Online Algebraic Geometry Seminar

This seminar is held online via Microsoft Teams. The current organisers are Alex Kasprzyk, Johannes Hofscheier, and Erroxe Etxabarri Alberdi. Previous organisers include Livia Campo.

To be added to the Team, please contact one of the organisers. For help joining a talk, please follow these instructions. Everybody is welcome.

Follow us on researchseminars.org or subscribe to the online calendar to be kept up-to-date about upcoming talks (also available in iCal format). Recordings of talks are available on our YouTube channel.

Note: All times are UK times.

Upcoming Talks

• • 9 May 2024, 10-11am
  • Mingchen Xia (IMJ-PRG)
  • Partial Okounkov bodies and toric geometry
  • Given a big line bundle L on a projective manifold, Lazarsfeld–Mustată and Kaveh–Khovanskii introduced method of constructing convex bodies associated with L. These convex bodies are known as Okounkov bodies. When L is endowed with a singular positive Hermitian metric h, I will explain how to construct smaller convex bodies from the data (L,h). These convex bodies play important roles in the study of the singularities of h. As an application, I will explain a non-trivial application in toric geometry due to Yi Yao.
  • Link to Event

Past Talks

• • 18 April 2024
  • Andrés Jaramillo Puentes (Duisburg-Essen)
  • Examples of enumerative problems for arbitrary fields
  • Over the complex numbers the solutions to enumerative problems are invariant: the number of solutions of a polynomial equation or polynomial system, the number of lines or curves in a surface, etc. Over the real numbers such invariance fails. However, the signed count of solutions may lead to numerical invariants: Descartes' rule of signs, Poincaré–Hopf theorem, real curve-counting invariants. Since many of these problems have a geometric nature, one may ask the same problems for arbitrary fields. Motivic homotopy theory allows to do enumerative geometry over an arbitrary base, leading to additional arithmetic and geometric information. The goal of this talk is to illustrate a generalized notion of sign that allows us to state a motivic version of classical problems: the number of lines on cubic surfaces, the Bézout theorem, and the curve-counting invariants.
  • Video (YouTube), Video (Vimeo)
• • 4 April 2024
  • Elena Denisova (Edinburgh)
  • Family 3–5 and δ-invariant of polarized del Pezzo surfaces
  • It is known that a smooth Fano variety admits a Kähler–Einstein metric if and only if it is K-polystable. For two-dimensional Fano varieties (del Pezzo surfaces) Tian and Yau proved that a smooth del Pezzo surface is K-polystable if and only if it is not a blow up of P² in one or two points. A lot of research was done for threefolds however, not everything is known and often the problem can be reduced to computing δ-invariant of (possibly singular) del Pezzo surfaces. In my talk, I will present an explicit example of such computation.
  • Video (YouTube), Video (Vimeo), Slides
• • 28 March 2024
  • Eduardo da Silva (Universite Paris Saclay)
  • Log Calabi–Yau geometry and Cremona maps
  • In the context of algebraic geometry, decomposition and inertia groups are special subgroups of the Cremona group which preserve a certain subvariety of Pⁿ as a set and pointwise, respectively. These groups were and still are classic objects of study in the area, with explicit descriptions in several instances. In the particular case where this fixed subvariety is a hypersurface of degree n+1, we have the notion of Calabi–Yau pair which allows us to use new tools to deal with the study of these groups and one of them is the so-called volume preserving Sarkisov Program. Using this approach we prove that an appropriate algorithm of the Sarkisov Program in dimension 2 applied to an element of the decomposition group of a nonsingular plane cubic is automatically volume preserving. From this, we deduce some properties of the (volume preserving) Sarkisov factorization of its elements. Regarding now a 3-dimensional context, we give a description of which toric weighted blowups of a point are volume preserving and among them, which ones will initiate a volume preserving Sarkisov link from a Calabi-Yau pair (P³,D) of coregularity 2. In this case, D is necessarily an irreducible normal quartic surface having canonical singularities. This last result enhances and extends the recent works of Guerreiro and Araujo, Corti and Massarenti in a log Calabi–Yau geometrical perspective, and it is a possible starting point to study the decomposition group of such quartics.
  • Video (YouTube), Video (Vimeo), Slides 1, Slides 2
• • 29 February 2024
  • Lisa Marquand (Courant Institute)
  • The defect of a cubic threefold
  • The defect of a cubic threefold with isolated singularities is a measure of the failure of Poincare duality, and also the failure to be Q-factorial. From the work of Cheltsov, a cubic threefold with only nodal singularities is Q-factorial if and only if there are at most 5 nodes. We investigate the defect of cubic threefolds with worse than nodal isolated singularities, and provide a geometric method to compute this global invariant. One can then compute the Mixed Hodge structure on the middle cohomology of the cubic threefold, in terms of the defect (a global invariant) and local invariants (Du Bois and Link invariants) determined by the singularity types. We then relate the defect to geometric properties of the cubic threefold, showing it is positive if and only if the cubic contains a plane or a rational normal cubic scroll. The focus of this work is to provide more insight into the existence of reducible fibers for compactified intermediate jacobian fibrations associated to a smooth (not necessarily general) cubic fourfold. This is joint work with Sasha Viktorova.
  • Video (YouTube), Video (Vimeo), Slides
• • 1 February 2024
  • Annamaria Ortu (Gothenburg)
  • Moduli of stable fibrations
  • Smooth fibrations between projective varieties can be thought of as both a generalisation of vector bundles and as a way of studying the behaviour of projective varieties in families. On holomorphic vector bundles, the Hitchin–Kobayashi correspondence establishes an equivalence between slope-stability and the existence of canonical connections, called Hermite–Einstein connections. A foundational result in the theory of vector bundles is the construction of a moduli space of stable vector bundles; such a moduli space can also be constructed analytically through the Hitchin–Kobayashi correspondence. On smooth fibrations we will define an analytic stability condition which we use to construct a moduli space of analytically stable smooth fibrations.
  • Video (YouTube), Video (Vimeo), Slides
• • 25 January 2024
  • Calla Tschanz (Jagiellonian)
  • Expansions for Hilbert schemes of points on semistable degenerations
  • Let X –> C be a projective family of surfaces over a curve with smooth general fibres and simple normal crossing singularity in the special fibre X₀. We construct a good compactification of the moduli space of relative length n zero-dimensional subschemes on X\X₀ over C\{0}. In order to produce this compactification we study expansions of the special fibre X₀ together with various GIT stability conditions, generalising the work of Gulbrandsen–Halle–Hulek who use GIT to offer an alternative approach to the work of Li–Wu for Hilbert schemes of points on simple degenerations. We construct stacks which we prove to be equivalent to the underlying stack of some choices of logarithmic Hilbert schemes produced by Maulik–Ranganathan.
  • Video (YouTube), Video (Vimeo), Slides
• • 18 January 2024
  • Swaraj Sridhar Parde (Michigan)
  • A Frobenius version of Tian's alpha invariant, and the F-signature of Fano varieties
  • The Alpha invariant of a complex Fano manifold was introduced by Tian to detect its K-stability, an algebraic condition that implies the existence of a Kähler–Einstein metric. Demailly later reinterpreted the Alpha invariant algebraically in terms of a singularity invariant called the log canonical threshold. In this talk, we will present an analog of the Alpha invariant for Fano varieties in positive characteristics, called the Frobenius-Alpha invariant. This analog is obtained by replacing "log canonical threshold" with "F-pure threshold", a singularity invariant defined using the Frobenius map. We will review the definition of these invariants and the relations between them. The main theorem proves some interesting properties of the Frobenius-Alpha invariant; namely, we will show that its value is always at most 1/2 and make connections to a version of local volume called the F-signature. Time permitting, we will also discuss the semicontinuity properties of the Frobenius-Alpha invariant.
  • Video (YouTube), Video (Vimeo), Slides
• • 14 December 2023
  • Kimoi Kemboi (Cornell)
  • Exceptional collections and window categories
  • The derived category of a variety is a crucial algebraic invariant with several profound implications on the geometry of the underlying variety. This talk will focus on a particular structure of derived categories called a full exceptional collection. We will discuss the landscape of full exceptional collections and its connections to geometry, then explore how to produce them for linear GIT quotients using ideas from "window" categories and equivariant geometry. As an example, we will consider a large class of linear GIT quotients by a reductive group of rank two, where this machinery produces full exceptional collections consisting of tautological vector bundles. This talk is based on joint work with Daniel Halpern-Leistner.
• • 7 December 2023
  • Haidong Liu (Sun Yat-sen)
  • On Miyaoka type and Kawamata–Miyaoka type inequalities
  • In the classification theory of varieties with nef anti-canonical divisors, Miyaoka type and Kawamata–Miyaoka type inequalities which concern about the relations between the first and second Chern classes play an important role. In this talk, I will show some recent progress on these inequalities and their application on the classification of 3-folds with nef anti-canonical divisors. Part of these works is jointed with Masataka Iwai and Chen Jiang, and part is jointed with Jie Liu.
  • Video (YouTube), Video (Vimeo), Slides
• • 30 November 2023
  • Jennifer Li (Princeton)
  • On the cone conjecture for log Calabi–Yau mirrors of Fano 3-folds
  • Let Y be a smooth projective 3-fold admitting a K3 fibration f:Y -> P¹ with -K_Y = f*O(1). We show that the pseudoautomorphism group of Y acts with finitely many orbits on the codimension one faces of the movable cone if H³(Y,C) = 0, confirming a special case of the Kawamata–Morrison–Totaro cone conjecture. In Coates–Corti–Galkin–Kasprzyk 2016, Przyjalkowski 2018, and Cheltsov–Przyjalkowski 2018, the authors construct log Calabi–Yau 3-folds with K3 fibrations satisfying the hypotheses of our theorem as the mirrors of Fano 3-folds.
  • Video (YouTube), Video (Vimeo), Slides
• • 23 November 2023
  • Thibaut Delcroix (Montpellier)
  • Fano spherical varieties of small dimension and rank
  • A spherical variety (X,G) is a normal complex algebraic variety X equipped with the action of a connected complex reductive group G such that a Borel subgroup B of G acts with an open dense orbit. The rank of (X,G) is the rank of the lattice of B-eigenvalues in the B-module of rational functions on X. I will present the classification of the 260 locally factorial Fano spherical varieties (X,G) of dimension four and of rank two or less, obtained in a joint work with Pierre-Louis Montagard. Those spherical varieties are described via combinatorial data, from which it is easy to read off geometric properties of the underlying variety X, such as the Picard rank, anticanonical degree, K-stability, etc.
  • Video (YouTube), Video (Vimeo), Slides
• • 16 November 2023
  • Chengxi Wang (UCLA)
  • Fano varieties with extreme behavior
  • It is attractive to classify Fano varieties with various types of singularities that originated from the minimal model program. For a Fano variety, the Fano index is the largest integer m such that the anti-canonical divisor is Q-linearly equivalent to m times some Weil divisor. For Fano varieties of various singularities, I show the Fano indexes can grow double exponentially with respect to the dimension. Those examples are also conjecturally optimal and have a close connection with Calabi–Yau varieties of extreme behavior.
  • Video (YouTube), Video (Vimeo), Slides
• • 10 November 2023
  • Lena Ji (Michigan)
  • Symmetries of Fano varieties
  • Prokhorov and Shramov showed that the BAB conjecture (later proven by Birkar) implies the Jordan property for automorphism groups of complex Fano varieties. This property in particular gives an upper bound on the size of semisimple groups acting faithfully on n-dimensional complex Fano varieties, and this bound only depends on n. We investigate the geometric consequences of an action by a large semisimple group – in particular the symmetric group. We give an effective upper bound on the size of these symmetric group actions, and we obtain optimal bounds for certain classes of varieties (toric varieties and Fano weighted complete intersections). Finally, we draw a connection between large symmetric actions and boundedness of varieties, by showing that the maximally symmetric Fano fourfolds form a bounded family. This work is joint with Louis Esser and Joaquín Moraga.
  • Video (YouTube), Video (Vimeo), Slides
• • 26 October 2023
  • Kathlén Kohn (KTH)
  • Rolling-shutter cameras & Kummer's classification of order-one line congruences
  • In this talk, we explain how algebraic geometry can be used to model and understand rolling-shutter cameras. Most consumer cameras today (e.g. in smartphones) use rolling shutters that do not capture an image at the same time but rather scan rapidly across the scene to be captured. When such a camera moves and rotates, the resulting picture can show the same 3D point several times, and straight lines in 3-space become higher-degree curves on the image. The set of light rays through such a camera form an algebraic surface in the Grassmannian of lines in projective 3-space. Kummer classified such surfaces (classically called line congruence) of order-one in 1866. We explain how Kummer's classification essentially characterizes all rolling-shutter cameras that see a generic 3D point exactly once. When such a camera takes a picture of a line in 3-space, the image is a high-degree curve. We compute that degree D in terms of the movement and rotation of the camera, and show that the image curve has multiplicity D-1 at one special point on the image plane. This talk is based on ongoing work with Marvin Hahn, Orlando Marigliano, and Tomas Pajdla.
  • Video (YouTube), Video (Vimeo), Slides
• • 19 October 2023
  • Carl Lian (Tufts)
  • Counting curves on P^r
  • We will explain a complete solution to the following problem. If (C,p₁,...,p_n) is a general curve of genus g and x₁,...,x_n are general points on P^r, then how many degree d maps f:C -> P^r are there with f(p_i)=x_i? These are the "Tevelev degrees" of projective space, which were previously known only when r=1, when d is large compared to g, or virtually in Gromov–Witten theory. Time permitting, we will also discuss some partial results when the conditions f(p_i)=x_i are replaced by conditions f(p_i) -> X_i, where the X_i are linear spaces of any dimension.
  • Video (YouTube), Video (Vimeo), Slides
• • 12 October 2023
  • Yuchen Liu (Courant Institute)
  • Moduli of boundary polarized Calabi–Yau pairs
  • While the theories of KSBA stability and K-stability have been successful in constructing compact moduli spaces of canonically polarized varieties and Fano varieties, respectively, the case of Calabi–Yau varieties remains less well understood. I will discuss a new approach to this problem in the case of boundary polarized Calabi–Yau pairs (X,D), i.e. X is a Fano variety and D is an anticanonical Q-divisor, in which we consider all semi-log-canonical degenerations. One challenge of this approach is that the moduli stack can be unbounded. Nevertheless, if we consider boundary polarized Calabi–Yau pairs as degenerations of P² with plane curves, we show that there exists a projective good moduli space despite the unboundedness. This is joint work with K. Ascher, D. Bejleri, H. Blum, K. DeVleming, G. Inchiostro, and X. Wang.
  • Video (YouTube), Video (Vimeo), Slides
• • 5 October 2023
  • Nawaz Sultani (Academia Sinica)
  • Gromov–Witten theory of non-convex complete intersections
  • For a convex complete intersection X, the Quantum Lefshetz Hyperplane theorem (QLHT) relates the Gromov–Witten (GW) invariants of X to those of the ambient space. This is most notably used in the proof of genus 0 mirror symmetry for complete intersections in toric varieties, since the invariants of the ambient toric variety are easier to compute. However, orbifold complete intersections are rarely convex, hence QLHT often fails even in genus 0. In this talk, I will showcase a method to compute the genus 0 GW invariants for orbifold complete intersections in stack quotients of the form [V // G], regardless of convexity conditions. The invariants computed by this method include all the invariants one expects of QLHT, even when QLHT fails. This talk will include results from joint works with Felix Janda (Notre Dame) and Yang Zhou (Fudan), and with Rachel Webb (Berkeley).
  • Video (YouTube), Video (Vimeo), Slides
• • 28 September 2023
  • Mikhail Ovcharenko (Steklov)
  • Modularity of Landau–Ginzburg models
  • In the past decades, there were proposed many different inter-related approaches to Mirror Symmetry for Fano varieties. The goal of this talk is to show that in the case of Fano threefolds these approaches are in harmony with each other. General anticanonical sections of a Fano threefold and general fibres of its Landau–Ginbzurg model are K3 surfaces, so it is natural to consider Mirror Symmetry for K3 surfaces as well. One of its most interesting forms is so called Dolgachev–Nikulin duality: for a lattice L it corresponds to a complete family of L-polarized K3 surfaces a complete family of L^*-polarized K3 surfaces, where L^* is a dual lattice. For any smooth Fano threefold X we show that the polarization of its general anticanonical section by Pic(X) is Dolgachev–Nikulin dual to the polarization of a general fibre F of its tame compactified toric Landau–Ginzburg model Z –> P¹ by the (explicitly constructed) lattice of monodromy invariants. Moreover, if the anticanonical class of X is very ample, we prove that the deformation space of pairs (Z,F) form a complete family of Pic(X)^*-polarized K3 surfaces. As a consequence, we obtain that for any such Fano threefold X the corresponding moduli space of Pic(X)^*-polarized K3 surfaces is uniruled. This is a joint work with Charles Doran, Andrew Harder, Ludmil Katzarkov, and Victor Przyjalkowski.
  • Video (YouTube), Video (Vimeo), Slides
• • 21 September 2023
  • Theodoros Papazachariou (Glasgow)
  • On divisorial stability of finite covers
  • Divisorial stability of a polarised variety is a stronger – but conjecturally equivalent – variant of uniform K-stability introduced by Boucksom-Jonsson. Whereas uniform K-stability is defined in terms of test configurations, divisorial stability is defined in terms of convex combinations of divisorial valuations on the variety. In this talk, I will give a quick account on divisorial stability, and then I will describe the behaviour of divisorial stability under finite group actions. In particular, I will show that equivariant divisorial stability of a polarised variety is equivalent to log divisorial stability of its quotient. I will then use this result to give a general construction of equivariantly divisorially stable polarised varieties. This is joint work with R. Dervan.
  • Video (YouTube), Video (Vimeo), Slides
• • 14 September 2023
  • Deniz Genlik (Ohio)
  • Higher genus Gromov–Witten theory of Cⁿ/Z_n: Holomorphic anomaly equations and crepant resolution
  • In this talk, we present certain results regarding the higher genus Gromov–Witten theory of Cⁿ/Z_n obtained by studying its cohomological field theory structure in detail. Holomorphic anomaly equations are certain recursive partial differential equations predicted by physicists for the Gromov–Witten potential of a Calabi–Yau threefold. We prove holomorphic anomaly equations for Cⁿ/Z_n for any n >= 3. In other words, we present a phenomenon of holomorphic anomaly equations in arbitrary dimensions, a result beyond the consideration of physicists. The proof of this fact relies on showing that the Gromov–Witten potential of Cⁿ/Z_n lies in a certain polynomial ring. Moreover, we prove an arbitrary genera crepant resolution correspondence for Cⁿ/Z_n by showing that its cohomological field theory matches with that of KP^n-1, where KP^n-1 is the total space of the canonical bundle of P^n-1. More precisely, we show that the Gromov–Witten potential of KP^n-1 also lies in a similar polynomial ring, and we show that it matches with the Gromov–Witten potential of Cⁿ/Z_n under an isomorphism of these polynomial rings. This talk is based on the joint works arXiv:2301.08389 and arXiv:2308.00780 with Hsian-Hua Tseng.
  • Video (YouTube), Video (Vimeo), Slides
• • 7 September 2023
  • Mauro Porta (Strasbourg)
  • Categorified Beauville–Laszlo theorem (and related problems)
  • Sheaves of Azumaya algebras were introduced by Grothendieck to represent classes in the cohomological Brauer group of schemes, i.e. Br(X) := H²_ét(X; G_m), along the same lines every class in H¹_ét(X; G_m) is representable by a line bundle on X. However, it turns out that not every class in Br(X) can be represented by a sheaf of Azumaya algebras, as shown in the case of Mumford's normal surface. In much more recent times, Toën introduced the notion of sheaf of derived Azumaya algebra, and proved that these objects represent even non-torsion classes in Br(X). In collaboration with Federico Binda we studied two problems related to derived Azumaya algebras: the Grothendieck existence and the Beauville–Laszlo theorems. In this talk, I will survey both questions and explain how our categorified approach allows to go beyond a classical injectivity result of Grothendieck. I will finish with a brief discussion of the consequences of categorified Beauville–Laszlo that will be the object of a future work.
  • Video (YouTube), Video (Vimeo), Slides
• • 31 August 2023
  • Yalong Cao (RIKEN)
  • From curve counting on Calabi–Yau 4-folds to quasimaps for quivers with potentials
  • I will start by reviewing an old joint work with Davesh Maulik and Yukinobu Toda on relating Gromov–Witten, Gopakumar–Vafa (in the sense of Klemm–Pandharipande) and stable pair invariants on compact Calabi–Yau 4-folds. For non-compact CY4 like local curves, similar invariants can be studied via the perspective of quasimaps to quivers with potentials. In a recent joint work with Gufang Zhao, we define a virtual count for such quasimaps and prove a gluing formula. Computations of examples will also be discussed.
  • Slides
• • 24 August 2023
  • Livia Campo (KIAS)
  • Flags on Fano 3-fold hypersurfaces
  • The existence of Kaehler–Einstein metrics on Fano 3-folds can be determined by studying some positive numbers called stability thresholds. K-stability is ensured if appropriate bounds can be found for these thresholds. An effective way to verify such bounds is to construct flags of point-curve-surface inside the Fano 3-folds. This approach was initiated by Abban–Zhuang, and allows us to restrict the computation of bounds for stability thresholds only on flags. We employ this machinery to prove K-stability of terminal quasi-smooth Fano 3-fold hypersurfaces. Many of these varieties had been attacked by Kim–Okada–Won using log canonical thresholds. In this talk I will tackle the remaining Fano hypersurfaces via Abban–Zhuang Theory.
  • Video (YouTube), Video (Vimeo), Slides
• • 3 August 2023
  • Alex Massarenti (Ferrara)
  • On the (uni)rationality problem for quadric bundles and hypersurfaces
  • A variety X over a field is unirational if there is a dominant rational map from a projective space to X. We will discuss the unirationality problem for quartic hypersurfaces and quadric bundles over a arbitrary field in the the perspective of the relation between unirationality and rational connectedness. We will prove unirationality of quadric bundles under certain positivity assumptions on their anti-canonical divisor. As a consequence we will get the unirationality of any smooth 4-fold quadric bundle over the projective plane, over an algebraically closed field, and with discriminant of degree at most 12.
  • Video (YouTube), Video (Vimeo), Slides
• • 27 July 2023
  • Andres Fernandez Herrero (Columbia)
  • Harder–Narasimhan theory for gauged maps
  • In this talk, I will discuss recent techniques developed to construct moduli spaces of decorated principal bundles on a fixed compact Riemann surface. Using these techniques, we construct a Harder–Narasimhan stratification, which can be used to obtain a generalization of the Verlinde formula in the context of decorated principal bundles. This talk is based on joint work with Daniel Halpern–Leistner.
  • Video (YouTube), Video (Vimeo), Slides
• • 20 July 2023
  • Sokratis Zikas (Poitiers)
  • On connected algebraic subgroups of groups of birational transformations
  • The problem of understanding the structure of the group of birational transformations Bir(X) of a projective variety X is an old one, with early results dating all the way back to the 19th century. In general Bir(X) does not admit the structure of an algebraic group; however one may study algebraic subgroups of it and how they relate to one another. In the last decade there has been a resurgence of results in this area, mainly due to the use of the modern machinery of the Minimal Model Program and the Sarkisov Program. In this talk I will present this modern framework as well as various results around the study of algebraic subgroups of Bir(X) for a Mori fiber space X.
  • Video (YouTube), Video (Vimeo), Slides
• • 13 July 2023, 3-4pm
  • Junyan Zhao (Illinois)
  • Moduli of curves of genus 6 and K-stability
  • In this talk, I will describe a way to study moduli of curves of small genus (eg. g=3,4,6) via K-stability. For instance, a general curve C of genus 6 can be embedded into the unique quintic del Pezzo surface X₅ as a divisor of class -2K_X5. Thus the K-moduli spaces of the pair (X₅, cC) are birational to the moduli of DM-stable curves M₆. On the other hand, X₅ can be embedded in P¹xP² as a divisor of class O(1,2), under which -2K_X is linearly equivalent to O_X(2,2). One can study the VGIT-moduli spaces in this setting. In this talk, I will compare these various compactifications of moduli spaces.
  • Video (YouTube), Video (Vimeo), Slides
• • 6 July 2023
  • Bruno Suzuki (São Paulo)
  • Birationally equivalent Landau–Ginzburg models on cotangent bundles and adjoint orbits
  • We show that the Lie potential on the minimal semisimple adjoint orbit of sl(n+1,C) coincides with toric potential on the cotangent bundle of Pⁿ. We then study the corresponding Landau–Ginzburg models in deformation families and give some examples of how the deformations affect the mirrors.
  • Video (YouTube), Video (Vimeo), Slides
• • 22 June 2023
  • Alexander Esterov (LIMS)
  • Bernstein–Kouchnirenko–Khovanskii with a symmetry
  • A generic polynomial f(x,y,z) with a prescribed Newton polytope defines a symmetric spatial curve f(x,y,z)=f(y,x,z)=0. We shall study its geometry, and classify the Newton polytopes for which this geometry is exceptional. As a motivating application, we shall classify generic one-parameter families of complex univariate polynomials, whose Galois group differs from the complete symmetric group. We shall see how some of these results conjecturally extend to higher dimensions and more complicated symmetries. This is based on joint work with Lionel Lang.
  • Video (YouTube), Video (Vimeo), Slides
• • 1 June 2023
  • Fei Si (BICMR)
  • K-moduli space of del Pezzo surface pairs
  • A K3 surfaces with anti-symplectic involution can be identified with a pair (X,C) consisting of a del Pezzo surface X with a curve C ~ −2K_X. Their moduli space has many compactifications from various perspectives. In this talk, we will discuss the compactifications from K-moduli theoretic side and its relation to Baily–Borel compactification from Hodge theoretic side. In particular, we will give an explicit description of K-moduli space P_c^K parametrizing K-polystable del Pezzo pairs (X,cC) under the framework of wall-crossing for K-moduli space due to Ascher–DeVleming–Liu. Moreover, we will show the K-moduli space P_c^K is isomorphic to certain log canonical model on Baily–Borel compactification of the moduli space of K3 surfaces with anti-symplectic involution. This can be viewed as another example of Hassett–Keel–Looijenga program proposed by Laza–O'Grady. This is based on joint work with Long Pan and Haoyu Wu.
  • Video (YouTube), Video (Vimeo), Slides
• • 25 May 2023
  • Kelly Jabbusch (Michigan)
  • The minimal projective bundle dimension and toric 2-Fano manifolds
  • In this talk we will discuss higher Fano manifolds, which are Fano manifolds with positive higher Chern characters. In particular we will focus on toric 2-Fano manifolds. Motivated by the problem of classifying toric 2-Fano manifolds, we will introduce a new invariant for smooth projective toric varieties, the minimal projective bundle dimension, m(X). This invariant m(X) captures the minimal degree of a dominating family of rational curves on X or, equivalently, the minimal length of a centrally symmetric primitive relation for the fan of X. We'll present a classification of smooth projective toric varieties with m(X) ≥ dim(X)-2, and show that projective spaces are the only 2-Fano manifolds among smooth projective toric varieties with m(X) equal to 1, dim(X)-2, dim(X)-1, or dim(X). This is joint work with Carolina Araujo, Roya Beheshti, Ana-Maria Castravet, Svetlana Makarova, Enrica Mazzon, and Nivedita Viswanathan.
  • Video (YouTube), Video (Vimeo), Slides
• • 18 May 2023
  • Antonio Trusiani (Toulouse)
  • A relative Yau–Tian–Donaldson conjecture and stability thresholds
  • On a Fano variety, the Yau–Tian–Donaldson correspondence connects the existence of Kähler–Einstein metrics to an algebro-geometric notion called K-stability. In the last decade, the latter has proved to be very valuable in algebraic geometry: for instance, it is used for the construction of moduli spaces. In the first part of the talk, partly motivated by the study of Kähler–Einstein metrics with prescribed singularities, a new relative K-stability notion will be introduced for a fixed smooth Fano variety. A particular focus will be given to motivations and intuitions, making a comparison with the log K-stability/log Kähler–Einstein metrics. The relative K-stability and the Kähler–Einstein metrics with prescribed singularities will then be related to each other through a Yau–Tian–Donaldson correspondence, which will be the core of the talk. An important role will be played by algebro-geometric valuative criteria, which will be also used to link the relative K-stability to the genuine K-stability.
  • Video (YouTube), Video (Vimeo), Slides
• • 11 May 2023
  • Daniel Bates (US Naval Academy)
  • Numerical methods for working with polynomial systems
  • Whether testing conjectures in algebraic geometry or trying to solve polynomial systems for some application, numerical methods are sometimes a useful alternative to well-known symbolic algorithms. This talk is intended to introduce some of the main tools of the field of numerical algebraic geometry, including homotopy continuation and the numerical irreducible decomposition. In particular, given a polynomial system, we will see how numerical methods can provide floating point approximations to points on each irreducible component of the corresponding complex variety. We will also visit a few recent uses of these methods and consider the benefits and drawbacks compared to exact, symbolic methods.
  • Video (YouTube), Video (Vimeo), Slides
• • 4 May 2023
  • Chuyu Zhou (EPFL)
  • On wall crossing for K-stability with multiple boundaries
  • In this talk, we will focus on a wall-crossing theory for log Fano pairs with multiple boundaries. As a key ingredient, we will present that the K-semistable domains are polytopes. This is based on a recent work arXiv:2302.13503.
  • Video (YouTube), Video (Vimeo), Slides
• • 27 April 2023
  • Aline Zanardini (Leiden)
  • Pencils of plane cubics revisited
  • In recent joint work with M. Hattori we have considered the problem of classifying linear systems of hypersurfaces (of a fixed degree) in some projective space up to projective equivalence via geometric invariant theory (GIT). And we have obtained a complete and explicit stability criterion. In this talk I will explain how this criterion can be used to recover Miranda’s description of the GIT stability of pencils of plane cubics.
  • Video (YouTube), Video (Vimeo), Slides
• • 20 April 2023
  • Lukas Braun (Freiburg)
  • Reductive quotients of klt varieties
  • In this talk, I will explain the proof of a recent result, obtained together with Daniel Greb, Kevin Langlois, and Joaquin Moraga, that reductive quotients of klt type varieties are of klt type. This generalizes and extends a classical result by Boutot, stating that these kinds of quotients preserve rational singularities. The statement was also well known in the case of finite groups. If time permits, I will also discuss several applications of our result, e.g. on quotients of Fano type varieties, good moduli spaces, and collapsing of homogeneous bundles.
  • Video (YouTube), Video (Vimeo), Slides
• • 13 April 2023
  • Louis Esser (UCLA)
  • Automorphisms of weighted projective hypersurfaces
  • Automorphism groups of smooth hypersurfaces in projective space are well studied in algebraic geometry. In this talk, I'll work in the more general setting of automorphism groups of quasismooth hypersurfaces in weighted projective space and consider the following questions: when are these groups linear? When are they finite, and if finite, how large can they get? What does the automorphism group of a very general hypersurface with given weights and degree look like? In each case, I'll generalise analogous results for ordinary projective hypersurfaces and explain how unexpected behaviour appears in the weighted setting.
  • Video (YouTube), Video (Vimeo), Slides
• • 12 April 2023
  • Maria Gioia Cifani (Roma Tre)
  • Reconstructing curves from their Hodge classes
  • Recently, Movasati and Sertöz pose several interesting questions about the reconstruction of a variety from its Hodge class. In particular they give the notion of a perfect class: the Hodge class of a variety X is perfect if its annihilator is a sum of ideals of varieties whose Hodge class is a nonzero rational multiple of that of X. I will report on a joint work with Gian Pietro Pirola and Enrico Schlensiger, in which we give an answer to some of these questions for curves: in particular, we show that the Hodge class of a smooth rational quartic on a surface of degree 4 is not perfect, and that the Hodge class of an arithmetically Cohen-Macaulay curve is always perfect. Moreover, I will give some results on the problem in higher dimension.
  • Video (YouTube), Video (Vimeo), Slides
• • 23 March 2023
  • Francesca Zaffalon (KU Leuven)
  • Toric degenerations of partial flag varieties via matching fields and combinatorial mutations
  • Toric degenerations are an important tool that can be used to analyze algebraic varieties as they allow us to understand a general variety via the geometry of their associated toric varieties. In this talk, I will show how to produce a new large family of toric degenerations of Grassmannians and (partial) flag varieties, whose combinatorics is governed by matching fields. Moreover, I will study the relations between polytopes associated to different toric degenerations of the same variety. This is done using the tool of combinatorial mutations, particular piecewise linear functions on polytopes. Finally, I will show how our methods can be used to compute new families of toric degenerations of small Grassmannians and flag varieties. This talk is based on joint work with Oliver Clarke and Fatemeh Mohammadi.
  • Video (YouTube), Video (Vimeo), Slides
• • 17 March 2023
  • Bernd Siebert (UT Austin)
  • Toward the logarithmic Hilbert scheme
  • Logarithmic geometry provides tools to work relative a normal crossings divisor, including normal crossings degenerations. I will report on work in progress with Mattia Talpo and Richard Thomas to define a natural logarithmic analogue of the ordinary Hilbert scheme. Immediate applications include induced good degenerations of Hilbert schemes of points. Our point of view also suggests a definition of tropical Hilbert schemes. One larger aim is to develop robust logarithmic methods to deal with coherent sheaves in maximal degenerations as they appear in mirror symmetry.
  • Video (YouTube), Video (Vimeo), Slides
• • 16 March 2023
  • Stefano Urbinati (Udine)
  • Mori Dream Pairs and C*-actions
  • The idea of the talk is that of giving a connection between "local" Mori theory and C*-actions. In particular, we construct and characterize a correspondence between Mori dream regions arising from small modifications of normal projective varieties and C*-actions on polarized pairs which are bordisms. This is joint work with Lorenzo Barban, Eleonora A. Romano and Luis E. Solá Conde.
  • Video (YouTube), Video (Vimeo), Slides
• • 9 March 2023, 10-11am
  • Tom Ducat (Durham)
  • Quartic surfaces up to volume preserving equivalence
  • We consider log Calabi–Yau pairs of the form (P³, D), where D is a quartic surface, up to volume-preserving equivalence. The coregularity of the pair (P³, D) is a discrete volume-preserving invariant c=0,1 or 2, and which depends on the nature of the singularities of D. We classify all pairs (P³, D) of coregularity c=0 or 1 up to volume preserving equivalence. In particular, if c=0 then we show that (P³, D) admits a volume preserving birational map onto a toric pair.
  • Video (YouTube), Video (Vimeo), Slides
• • 2 March 2023
  • Ayush Kumar Tewari (Ghent)
  • Forbidden patterns in tropical planar curves and panoptigons
  • Tropical curves in R² correspond to metric planar graphs but not all planar graphs arise in this way. We describe several new classes of graphs that cannot occur. For instance, this yields a full combinatorial characterization of the tropically planar graphs of genus at most six. We also define a special family of lattice polytopes namely panoptigons and enumerate all possible panoptigons under mild lattice width constraints and show how they can be used to find a forbidden pattern in tropical planar curves. We also will discuss some possible applications of the classification of panoptigons and ongoing work on suitable generalizations. This talk is based on work in Tewari (2022) and joint work with Michael Joswig (2020) and Ralph Morrison (2021).
  • Video (YouTube), Video (Vimeo), Slides
• • 22 February 2023
  • Fenglong You (ETH-Zurich)
  • Relative quantum cohomology under birational transformations
  • I will talk about how relative quantum cohomology, defined by Tseng–You and Fan–Wu–You, varies under birational transformations. Relation with FJRW theory and extremal transitions of absolute Gromov–Witten theory will also be discussed.
  • Video (YouTube), Video (Vimeo), Slides
• • 16 February 2023
  • Duc-Khanh Nguyen (Albany)
  • A generalization of the Murnaghan-Nakayama rule for K-k-Schur and k-Schur functions
  • We introduce a generalization of K-k-Schur functions and k-Schur functions via the Pieri rule. Then we obtain the Murnaghan–Nakayama rule for the generalized functions. The rule are described explicitly in the cases of K-k-Schur functions and k-Schur functions, with concrete descriptions and algorithms for coefficients. Our work recovers the result of Bandlow, Schilling, and Zabrocki for k-Schur functions, and explains it as a degeneration of the rule for K-k-Schur functions. In particular, many other special cases promise to be detailed in the future.
  • Video (YouTube), Video (Vimeo), Slides
• • 9 February 2023
  • Iacopo Brivio (NCTS)
  • Invariance of plurigenera and KSBA moduli in positive and mixed characteristic
  • A famous theorem by Siu states that plurigenera are invariant under smooth deformations for complex projective manifolds, a result which is a cornerstone of higher dimensional moduli theory. In this talk we will explore some examples showing that Siu's theorem fails in positive and mixed characteristic, then discuss the implications at the level of moduli theory, as well as some related questions.
  • Video (YouTube), Video (Vimeo), Slides
• • 2 February 2023
  • Yaoxiong Wen (KIAS)
  • Mirror symmetry for the parabolic G-Higgs bundle, from local to global
  • Motivated by geometric Langlands, we initiate a program to study the mirror symmetry for the moduli space of parabolic G-Higgs bundles. This talk will focus on G=Sp_2n and its Langlands dual SO_2n+1. Our goal is to prove the SYZ mirror symmetry and topological mirror symmetry (TMS). The parabolic structure of the parabolic Higgs bundle is related to the nilpotent orbit closure. So we need to first figure out the mirror pair for nilpotent orbits. Classically, there is a famous Springer duality between special orbits. Therefore, it is natural to speculate that the mirror symmetry we seek may coincide with Springer duality in the context of special orbits. Unfortunately, such a naive statement fails. To remedy the situation, together with Prof. Ruan and Prof. Fu (arXiv:2207.10533), we propose a conjecture which asserts the mirror symmetry for certain parabolic/induced covers of special orbits. Then, we prove the conjecture for Richardson orbits and obtain certain partial results in general. After understanding the mirror parabolic structures, together with W. He, X. Su, B. Wang, X. Wen, we are working in progress to prove the SYZ and TMS for the moduli space of parabolic Sp_2n / SO_2n+1-Higgs bundles with dual parabolic structures.
  • Video (YouTube), Video (Vimeo), Slides
• • 26 January 2023
  • Lucie Devey (Frankfurt and Grenoble)
  • Stability of toric vector bundles in terms of parliaments of polytopes
  • Given any toric vector bundle, we may construct its parliament of polytopes. This is a generalization of the Newton polytope (or moment polytope) of a toric line bundle. This object contains a huge amount of information about the original bundle: notably on its global sections and its positivity. We can also easily know if the toric bundle is (semi-/poly-)stable with respect to any polarisation. I will give a combinatorial visualisation of stability of toric vector bundles.
  • Video (YouTube), Video (Vimeo), Slides
• • 8 December 2022
  • Tiago Duarte Guerreiro (Essex)
  • On toric Sarkisov Links from P⁴
  • According to the Sarkisov Program, a birational map between a Fano variety of Picard rank one and a Mori fibre space can be decomposed as a finite sequence of Elementary Sarkisov Links starting with the blowup of a centre. Hence, it is natural to try to understand the latter maps explicitly. In this talk we explain how to describe all possible toric Elementary Sarkisov Links starting with the blowup of a point in P⁴.
  • Video (YouTube), Video (Vimeo), Slides
• • 28 November 2022
  • Rob Silversmith (Warwick)
  • Cross-ratios and perfect matchings
  • Given a bipartite graph G (subject to a constraint), the "cross-ratio degree" of G is a non-negative integer invariant of G, defined via a simple counting problem in algebraic geometry. I will discuss some natural contexts in which cross-ratio degrees arise. I will then present a perhaps-surprising upper bound on cross-ratio degrees in terms of counting perfect matchings. Finally, time permitting, I may discuss the tropical side of the story.
  • Video (YouTube), Video (Vimeo)
• • 17 November 2022
  • Vasiliki Petrotou (Hebrew University of Jerusalem)
  • Tom & Jerry triples and the 4-intersection unprojection formats
  • Unprojection is a theory in Commutative Algebra due to Miles Reid which constructs and analyses more complicated rings from simpler ones. The talk will be about two new formats of unprojection which we call Tom & Jerry triples and 4-intersection format respectively. The motivation is to construct codimension 6 Gorenstein rings starting from codimensions 3 and 2 respectively. As an application we will construct three families of codimension 6 Fano 3-folds in weighted projective space which appear in the Graded Ring Database.
  • Video (YouTube), Video (Vimeo), Slides
• • 10 November 2022
  • Joey Palmer (Illinois)
  • Integrable systems with S¹-actions and the associated polygons
  • Semitoric systems are a type of four-dimensional integrable system which admit a global S¹-action; these systems were classified by Pelayo and Vu Ngoc in 2011, generalizing the classification of toric integrable systems and making use of an invariant called a "semitoric polygon". I will present some results about bifurcations of such systems, and show how this can be used to construct explicit examples of such systems associated to certain given semitoric polygon. Time permitting, I will also discuss how hypersemitoric systems, a generalization of semitoric systems, appear in this context. Some of the results I will present are joint with Yohann Le Floch and Sonja Hohloch.
  • Video (YouTube), Video (Vimeo), Slides
• • 3 November 2022
  • Julia Schneider (EPFL)
  • Birational maps of Severi-Brauer surfaces, with applications to Cremona groups of higher rank
  • Cremona groups are groups of birational transformations of a projective space. Their structure depends on the dimension and the field. In this talk, however, we will first focus on birational transformations of (non-trivial) Severi-Brauer surfaces, that is, surfaces that become isomorphic to the projective plane over the algebraic closure of K. Such surfaces do not contain any K-rational point. We will prove that if such a surface contains a point of degree 6, then its group of birational transformations is not generated by elements of finite order as it admits a surjective group homomorphism to the integers. As an application, we use this result to study Mori fiber spaces over the field of complex numbers, for which the generic fiber is a non-trivial Severi-Brauer surface. We prove that any group of cardinality at most the one of the complex numbers is a quotient of the Cremona group of rank 4 (and higher). This is joint work with Jérémy Blanc and Egor Yasinsky.
  • Video (YouTube), Video (Vimeo), Slides
• • 20 October 2022
  • Alex Abreu (Fluminense Federal University)
  • Wall-crossing of Brill-Noether cycles in compactified Jacobians
  • We will discuss an explicit graph formula, in terms of boundary strata classes, for the wall-crossing of universal (over the moduli space of curves) Brill-Noether classes. More precisely, fix two stability conditions for universal compactified Jacobians that are on different sides of a wall in the stability space. Then we can compare the two universal Brill-Noether classes on the two compactified Jacobians by pulling one of them back along the (rational) identity map. The calculation involves constructing a resolution by means of subsequent blow-ups. This is joint with Nicola Pagani.
  • Video (YouTube), Video (Vimeo)
• • 13 October 2022
  • Luca Ugaglia (Palermo)
  • Seshadri constants of toric surfaces
  • In this talk, after introducing Seshadri constants of projective surfaces and some known results, I will focus on the case of toric projective surfaces associated to lattice polygons. I will prove some relations between the rationality of Seshadri constants and the geometry of the polygon, and I will present some possible applications to the case of weighted projective planes. This is based on a joint work with Antonio Laface.
  • Video (YouTube), Video (Vimeo), Slides
• • 6 October 2022
  • Aimeric Malter (Birmingham)
  • A derived equivalence of the Libgober-Teitelbaum and Batyrev-Borisov mirror constructions
  • In this talk I will demonstrate how Variations of Geometric Invariant Theory can be used to provide a derived equivalence between complete intersections in toric varieties. I will illustrate this by proving the derived equivalence of two mirror constructions, due to Libgober-Teitelbaum and Batyrev-Borisov.
  • Video (YouTube), Video (Vimeo), Slides
• • 29 September 2022
  • Luca Tasin (Milano)
  • Sasaki-Einstein metrics on spheres
  • It is a classical problem in geometry to construct new metrics on spheres. I will report on a joint work with Yuchen Liu and Taro Sano in which we construct infinitely many families of Sasaki-Einstein metrics on odd-dimensional spheres that bound parallelizable manifolds, proving in this way conjectures of Boyer-Galicki-Kollár and Collins-Székelyhidi. The construction is based on showing the K-stability of certain Fano weighted orbifold hypersurfaces.
  • Video (YouTube), Video (Vimeo), Slides
• • 22 September 2022
  • Gianluca Occhetta (Trento)
  • Maximal disjoint Schubert cycles in Rational Homogeneous spaces
  • In 1974 Tango proved that there are no non-constant morphisms from Pⁿ to the Grassmannian G(l,m) if n > m; similar results were later obtained for morphisms from other Fano manifolds to Grassmannians. In this talk I will present the following generalization of these results: if X and Y are rational homogeneous manifold obtained as quotients of classical groups G_X and G_Y of the same type and rk(G_X) > rk(G_Y) then there are no non-constant morphisms from X to Y. The key ingredient of the proof is the determination of the effective good divisibility of rational homogeneous manifolds of classical type, that is, the greatest integer s such that two effective cycles in the Chow ring whose sum of codimensions is s have nonzero intersection. This talk is based on a joint work with R. Muñoz and L.E. Solá Conde.
  • Video (YouTube), Video (Vimeo), Slides
• • 15 September 2022
  • Nathan Reading (North Carolina)
  • Scatter, cluster, scatter, model
  • Cluster algebras were invented/discovered in order to understand total positivity. But almost immediately, mathematicians (and later physicists) started finding connections between the combinatorics/geometry/algebra of cluster algebras and other areas of mathematics and physics. Most relevant for this talk are two connections: In one direction, the theory of scattering diagrams (mirror symmetry/Donaldson-Thomas theory/integrable systems) has been applied to prove key structural results about cluster algebras. In the other direction, certain cluster algebras seem to be relevant to the computation of scattering amplitudes in physics. The title of this talk is also an outline. I will introduce scattering diagrams, then introduce cluster algebras, and connect the two. Then I will give a brief, naïve summary of the observed connections between cluster algebras and scattering amplitudes, to motivate the idea that a physicist might be interested in combinatorial models for cluster algebras/scattering diagrams. I will conclude with a survey of the state of research on these combinatorial models, focusing on the models that I have worked most closely with.
  • Video (YouTube), Video (Vimeo), Slides
• • 1 September 2022
  • Alfredo Nájera Chávez (UNAM)
  • Newton–Okounkov bodies and minimal models of cluster varieties
  • I will explain a general procedure to construct Newton–Okounkov bodies for a certain class of (partial) compactifications of cluster varieties. This class consists of the (partial) minimal models of cluster varieties with enough theta functions. This construction applies for example to Grassmannians and Flag varieties, among others. Our construction depends on a choice of torus in the atlas of the cluster variety and the associated Newton–Okounkov body lives inside a real vector space. Time permitting, I will explain how to compare the Newton–Okounkov bodies associated with different tori and elaborate on the "intrinsic Newton–Okounkov body", which is an object that does not depend on the choice of torus and lives inside the real tropicalization of the mirror cluster variety. This is based on upcoming work with Lara Bossinger, Man-Wai Cheung and Timothy Magee.
  • Video (YouTube), Video (Vimeo), Slides
• • 25 August 2022
  • Tristan Hübsch (Howard)
  • Laurent Smoothing, Turin Degenerations and Mirror Symmetry
  • Calabi–Yau hypersurfaces in toric spaces of general type (encoded by certain non-convex polytopes) are degenerate but may be smoothed by rational anticanonical sections. Nevertheless, gauged linear sigma model phases and an increasing number of their classical and quantum data are just as computable as for their siblings encoded by reflexive polytopes, and they all have transposition mirrors. Showcasing Calabi–Yau hypersurfaces in Hirzebruch scrolls shows this class of constructions to be infinitely vast, yet amenable to several well-founded algebro-geometric methods of analysis. This talk will include joint work with Per Berglund, as reported in part: arXiv:1606.07420, arXiv:1611.10300 and arXiv:2205.12827.
  • Video (YouTube), Video (Vimeo), Slides
• • 11 August 2022
  • Felipe Espreafico (IMPA)
  • Gauss–Manin Connection in Disguise and Mirror Symmetry
  • In this talk, we aim to explain what the Gauss–Manin Connection in Disguise program is and why it is important. The idea is to construct objects which behave similarly to modular forms using the Gauss–Manin connection associated to a family of varieties with fixed topological data. We focus on the applications to Mirror Symmetry, especially the relations with Gromov–Witten invariants and the periods of the mirror quintic family. Among them, I will explain my results for the open string Mirror Symmetry and open Gromov–Witten invariants.
  • Video (YouTube), Video (Vimeo), Slides
• • 4 August 2022
  • Swarnava Mukhopadhyay (Tata Institute)
  • Graph potentials and mirrors of moduli of rank two bundles on curves
  • Graph potentials are Laurent polynomials associated to (colored) trivalent graphs that were introduced in a joint work with Belmans and Galkin. They naturally appear as Newton polynomials of natural toric degenerations of the moduli space of rank two bundles. In this talk we will first discuss how graph potentials compute quantum periods of the moduli space M of rank two bundles with fixed odd degree determinant and hence can be regarded as a partial mirror to M. From the view point of mirror symmetry, we will show how the critical value decomposition of graph potentials provides evidence for the conjectural semiorthogonal decomposition of D^bCoh(M). If time permits we will also discuss a formula to efficiently compute the periods of graph potential via a TQFT. This is a joint work with Pieter Belmans and Sergey Galkin.
  • Video (YouTube), Video (Vimeo), Slides
• • 28 July 2022
  • Luca Schaffler (Roma Tre University)
  • Boundary divisors in the compactification by stable surfaces of moduli of Horikawa surfaces
  • Smooth minimal surfaces of general type with K²=1, p_g=2, and q=0 constitute a fundamental example in the geography of algebraic surfaces, and the 28-dimensional moduli space M of their canonical models admits a modular compactification M via the minimal model program. We describe eight new irreducible boundary divisors in such compactification parametrizing reducible stable surfaces. Additionally, we study the relation with the GIT compactification of M and the Hodge theory of the degenerate surfaces that the eight divisors parametrize. This is joint work in progress with Patricio Gallardo, Gregory Pearlstein, and Zheng Zhang.
  • Video (YouTube), Video (Vimeo), Slides
• • 21 July 2022
  • Pierrick Bousseau (ETH Zürich)
  • Fock–Goncharov dual cluster varieties and Gross–Siebert mirrors
  • Cluster varieties come in pairs: for any X-cluster variety there is an associated Fock–Goncharov dual A-cluster variety. On the other hand, in the context of mirror symmetry, associated with any log Calabi–Yau variety is its mirror dual, which can be constructed using the enumerative geometry of rational curves in the framework of the Gross–Siebert program. I will explain how to bridge the theory of cluster varieties with the algebro-geometric framework of Gross–Siebert mirror symmetry, and show that the mirror to the X-cluster variety is a degeneration of the Fock–Goncharov dual A-cluster variety. To do this, we investigate how the cluster scattering diagram of Gross–Hacking–Keel–Kontsevich compares with the canonical scattering diagram defined by Gross–Siebert to construct mirror duals in arbitrary dimensions. This is joint work with Hülya Argüz.
  • Video (YouTube), Video (Vimeo), Slides
• • 14 July 2022
  • Léonard Pille-Schneider (Paris)
  • Degenerations of Calabi–Yau manifolds and integral affine geometry
  • Let X -> D^* be a maximal degeneration of n-dimensional Calabi–Yau varieties over the punctured disk. The SYZ conjecture, motivated by mirror symmetry, predicts that the general fiber X_t admits a Lagrangian torus fibration f_t:X_t -> B onto a base B of real dimension n, and that as t -> 0 the variety X_t endowed with its Ricci-flat Kähler metric collapses to the space B, endowed with a Z-affine structure. The goal of this talk is to explain how to construct the space B with its extra structures using non-archimedean geometry. In particular, in the case of Fermat threefolds in P⁴, using the toric geometry of the ambient space, we are able to construct a non-archimedean SYZ fibration inducing on B the affine structure naturally induced by the Gromov–Hausdorff convergence recently proved by Yang Li. This is based on work joint with Enrica Mazzon.
  • Video (YouTube), Video (Vimeo), Slides
• • 8 July 2022
  • Chen Jiang (SCMS)
  • Explicit boundedness of canonical Fano 3-folds
  • Motivated by the classification of canonical Fano 3-folds, we are interested in boundedness results on different kinds of canonical Fano 3-folds, such as anticanonical systems, indices, degrees, and so on. I will summarize known results with recent progress, such as the explicit upper bound of anitcanonical volumes and the effective birationality of anticanonical systems (based on joint works with Yu Zou) and some open problems.
  • Video (YouTube), Video (Vimeo), Slides
• • 7 July 2022
  • Elisa Postinghel (Trento)
  • The geometry of Weyl orbits on blow-ups of projective spaces
  • Linear systems of divisors on blow-ups of projective spaces in points in general positions are connected to certain polynomial interpolation problems. While for the case of plane curves and of surfaces in 3-space there are conjectures, although long standing, formulated by M. Nagata, B. Segre and others, in the higher dimensional case we are in the dark. However, when the number of points is not too large and the blow-ups are Mori dream spaces, an action of the Weyl group on cycles of any codimension governs the birational behaviour of the space on the one hand, and the stable base locus of divisors on the other hand, and it yields a solution to the interpolation problem. Joint work with C. Brambilla, O. Dumitrescu and L. Santana Sánchez.
  • Video (YouTube), Video (Vimeo), Slides
• • 23 June 2022
  • Andreas Bäuerle (Tübingen)
  • Gorenstein Fano 3-folds of Picard number 1 with a 2-torus action
  • We classify the non-toric, Q-factorial, log terminal, Gorenstein Fano threefolds of Picard number one that admit an effective action of a two-dimensional torus.
  • Slides
• • 26 May 2022
  • Fei Xie (Edinburgh)
  • Residual categories of quadric surface bundles
  • The residual category (or the Kuznetsov component) of a quadric surface bundle is the non-trivial component in the derived category. It is equivalent to the twisted derived category of a double cover over the base when the quadric surface bundle has simple degeneration (fibers have corank at most 1). I will consider quadric surface bundles with fibers of corank at most 2 and describe their residual categories as (twisted) derived categories of some scheme in two situations: (1) when the bundle has a smooth section; (2) when the total space is smooth and the base is a smooth surface. The results can be applied to describe the residual categories of a (partial) resolution of nodal quintic del Pezzo threefolds, cubic fourfolds containing a plane and certain complete intersections of quadrics.
  • Video (YouTube), Video (Vimeo), Slides
• • 19 May 2022
  • Kiumars Kaveh (Pittsburgh)
  • Buildings as classifying spaces for toric principal bundles
  • A building is a certain infinite combinatorial object (abstract simplicial complex) associated to a (semisimple) linear algebraic group which encodes the relative position of maximal tori and parabolic/parahoric subgroups in it. After an introduction to buildings and discussing some examples from linear algebra, I will talk about some recent results on classification of torus equivariant principal G-bundles on toric varieties (over a field) and toric schemes (over a discrete valuation ring). These are extensions of Klyachko's classification of torus equivariant vector bundles on toric varieties. For this we introduce the notions of "piecewise linear map" to the Tits building and "piecewise affine map" to the Bruhat-Tits building of a linear algebraic group. This is joint work with Chris Manon (Kentucky) and Boris Tsvelikhovsky (Pittsburgh).
  • Video (YouTube), Video (Vimeo), Slides
• • 12 May 2022
  • Noah Arbesfeld (Kavli IPMU)
  • Descendent series for Hilbert schemes of points on surfaces
  • Structure often emerges from Hilbert schemes of points on varieties when the underlying variety is fixed but the number of points parametrized varies. Some examples of such structure come from integrals of tautological bundles, which arise in geometric and physical computations. When compiled into generating series, these integrals display interesting functional properties. I will give an overview of results on such series; the focus will be on K-theoretic descendent series for Hilbert schemes on surfaces, certain series formed from holomorphic Euler characteristics of tautological bundles. In particular, I will explain how to see that the K-theoretic descendent series are expansions of rational functions.
  • Video (YouTube), Video (Vimeo), Slides
• • 5 May 2022
  • Veronica Fantini (IHÉS)
  • Enumerative geometry in the extended tropical vertex group
  • The extended tropical vertex group is a pro-nilptotent Lie group, which has been introduced in arXiv:1912.09956 studying the relationship between scattering diagrams and infinitesimal deformations of holomorphic pairs. Scattering diagrams were introduced by Kontsevich and Soibelman in the context of mirror symmetry. They are defined algebraically, in terms of pro-nilpotent Lie groups, but in many applications they have a combinatorial structure which encodes enumerative geometric data (as Donaldson–Thomas invariants, Gromov–Witten invariants,...). In particular, Gross, Pandharipande and Siebert showed how to compute genus zero log Gromov–Witten invariants for P² via scattering diagrams in the so called tropical vertex group. In this talk, I will discuss a possible generalization regarding how to compute genus zero relative Gromov–Witten invariants for toric P² using scattering diagrams in the extended tropical vertex group.
  • Video (YouTube), Video (Vimeo), Slides
• • 28 April 2022
  • Nikolaos Tsakanikas (Saarbrücken)
  • On the existence of minimal models for generalized pairs
  • I will discuss recent progress on the existence of minimal models and Mori fiber spaces for generalized pairs. In particular, I will explain the close relationship between the existence of minimal models and the existence of weak Zariski decompositions for generalized pairs. This is joint work with Vladimir Lazić.
  • Video (YouTube), Video (Vimeo), Slides
• • 21 April 2022
  • Nivedita Viswanathan (Loughborough)
  • On K-stability of some singular del Pezzo surfaces
  • There has been a lot of development recently in understanding the existence of Kahler-Einstein metrics on Fano manifolds due to the Yau-Tian-Donaldson conjecture, which gives us a way of looking at this problem in terms of the notion of K-stability. In particular, this problem is solved in totality for smooth del Pezzo surfaces by Tian. For del Pezzo surfaces with quotient singularities, there are partial results. In this talk, we will consider singular del Pezzo surfaces of indices 2 and 3, which are quasi-smooth, well-formed hypersurfaces in weighted projective space, and understand what we can say about their K-stability. This is joint work with In-Kyun Kim and Joonyeong Won.
  • Video (YouTube), Video (Vimeo), Slides
• • 14 April 2022
  • Zakarias Sjöström Dyrefelt (Aarhus-AIAS)
  • Constant scalar curvature and Kähler manifolds with nef canonical bundle
  • Given a compact Kähler manifold it is a classical question, related to K-stability, whether it admits a Kähler metric of constant scalar curvature (cscK metric for short). In this talk we prove that there always exist cscK metrics on compact Kähler manifolds with nef canonical bundle, thus on all smooth minimal models, and also on the blowup of any such manifold. This confirms an expectation of Jian-Shi-Song and extends well-known results of Aubin and Yau to the nef case, giving a large new class of examples of cscK manifolds. The tools used are from the variational approach in Kähler geometry, and some related results on stability thresholds and Donaldson's J-equation are discussed along the way.
  • Slides
• • 7 April 2022
  • Jeff Hicks (Edinburgh)
  • Pretalk: Mirror Symmetry and Lagrangian torus fibrations
  • Mirror symmetry is a predicted equivalence between certain aspects of algebraic geometry and symplectic geometry. The Strominger–Yau–Zaslow conjecture proposes that this equivalence appears on pairs of algebraic and symplectic spaces which have dual torus fibrations. In this pretalk, we look at a first example: the complex torus which is fibered by real tori, and the cotangent bundle of the real torus. We'll see how both geometries can be related to affine geometry on real n-dimensional space.
  • Video (YouTube), Video (Vimeo), Slides
  • Realizing tropical curves via mirror symmetry
  • The tropicalization map associates to each curve in the algebraic n-torus a piecewise linear object (tropical curve) in real n-dimensional space. Given a tropical curve, a natural question is if it can arise as the tropicalization of some algebraic curve. If this is the case we say that the tropical curve is realizable. Determining good realizability criteria for tropical curves remains an important part of tropical geometry since Mikhalkin provided examples of non-realizable tropical curves. We explore the following strategy for realizing tropical curves:
    • (1) Produce a Lagrangian submanifold of the cotangent bundle of the torus whose moment map projection approximates the tropical curve;
    • (2) Use homological mirror symmetry to obtain a mirror algebraic sheaf;
    • (3) Show that the tropicalization of the support of this sheaf is the original tropical curve.
    We will give full answers to (1) and (3), and explain why (2) is fairly subtle. As applications, we will obtain some new and known realizability statements for tropical curves.
  • Video (YouTube), Video (Vimeo), Slides
• • 31 March 2022
  • Franco Rota (Glasgow)
  • Full exceptional collection for anticanonical log del Pezzo surfaces
  • The homological mirror symmetry conjecture predicts a correspondence between the derived category of coherent sheaves of a variety and the symplectic data (packaged in the Fukaya category) of its mirror object. Motivated by this, we construct exceptional collections for (the smooth stacks associated with) a family of log del Pezzo surfaces known as the Johnson-Kollar series. These surfaces have quotient, non-Gorenstein, singularities. Thus, our computation will include on the one hand an application of the special McKay correspondence, and on the other the study of their minimal resolutions, which are birational to a degree 2 del Pezzo surface. This is all joint work with Giulia Gugiatti.
  • Video (YouTube), Video (Vimeo), Slides
• • 24 March 2022
  • Egor Yasinsky (École Polytechnique)
  • Birational involutions of the projective plane
  • Birational involutions of the projective plane (or, equivalently, automorphisms of the field of rational functions in two variables of order 2) were studied already by the Italian school of algebraic geometry — Bertini, Castelnuovo, and Enriques. However, their explicit and complete description was obtained by Beauville and Bayle only in 2000 and only in the case of a complex projective plane. It turns out that for planes over algebraically non-closed fields the situation is much more complicated. In the first part of the talk, I will review what is known about birational involutions of projective planes over various fields. In the second part, I will talk about the joint work with I. Cheltsov, F. Mangolt and S. Zimmerman, in which we classified birational involutions of the real projective plane.
  • Video (YouTube), Video (Vimeo), Slides
• • 17 March 2022
  • Alessio Borzì (Warwick)
  • Weierstrass sets on finite graphs
  • Weiestrass points and Weierstrass semigroups are classical objects of study in Algebraic Geometry. The problem of determining which semigroups arise as Weierstrass semigroups of a curve goes back to Hurwitz in 1893. After the advent of tropical geometry, a divisor theory on graphs was developed by Baker and Norine, and later extended to metric graphs (namely, abstract tropical curves) by Gathmann and Kerber, and Mikhalkin and Zharkov. In this talk we present two natural tropical analogues of Weierstrass semigroups on graphs, called rank and functional Weierstrass sets, first appeared in a work of Kang, Matthews and Peachey. We present some results on these two objects and their interplay.
  • Video (YouTube), Video (Vimeo), Slides
• • 3 March 2022
  • Qaasim Shafi (Imperial)
  • Logarithmic Toric Quasimaps
  • Quasimaps provide an alternate curve counting system to Gromov–Witten theory, related by wall-crossing formulas. Relative (or logarithmic) Gromov–Witten theory has proved useful for constructions in mirror symmetry, as well as for determining ordinary Gromov–Witten invariants via the degeneration formula. I’ll discuss how to build a theory of logarithmic quasimaps in the toric case, some restrictions, and why one might want to do so.
  • Video (YouTube), Video (Vimeo), Slides
• • 24 February 2022
  • Jarosław Buczyński (Polish Academy of Sciences)
  • Fujita vanishing, sufficiently ample line bundles, and cactus varieties
  • For a fixed projective manifold X, we say that a property P(L) (where L is a line bundle on X) is satisfied by sufficiently ample line bundles if there exists a line bundle M on X such that P(L) hold for any L with L-M ample. I will discuss which properties of line bundles are satisfied by the sufficiently ample line bundles - for example, can you figure out before the talk, whether a sufficiently ample line bundle must be very ample? A basic ingredient used to study this concept is Fujita's vanishing theorem, which is an analogue of Serre's vanishing for sufficiently ample line bundles. At the end of the talk I will define cactus varieties (an analogue of secant varieties) and sketch a proof that cactus varieties to sufficiently ample embeddings of X are (set-theoretically) defined by minors of matrices with linear entries. The topic is closely related to conjectures of Eisenbud-Koh-Stillman (for curves) and Sidman-Smith (for any varieties). The new ingredients are based on joint work in preparation with Weronika Buczyńska and Łucja Farnik.
  • Video (YouTube), Video (Vimeo), Slides
• • 10 February 2022
  • Ananyo Dan (Sheffield)
  • McKay correspondence for isolated Gorenstein singularities
  • The McKay correspondence is a (natural) correspondence between the (non-trivial) irreducible representations of a finite subgroup G of SL(2,C) and the irreducible components of the exceptional divisor of a minimal resolution of the associated quotient singularity C²//G. A geometric construction for this correspondence was given by González-Sprinberg and Verdier, who showed that the two sets also correspond bijectively to the set of indecomposable reflexive modules on the quotient singularity. This was generalised to higher dimensional quotient singularities (i.e., quotient of Cⁿ by a finite subgroup of SL(n,C)) by Ito-Reid, where the above sets were substituted by certain smaller subsets. It was further generalised to more general quotient singularities by Bridgeland-King-Reid, Iyama-Wemyss and others, using the language of derived categories. In this talk, I will survey past results and discuss what happens for the isolated Gorenstein singularities case (not necessarily a quotient singularity). If time permits, I will discuss applications to Matrix factorization. This is joint work in progress with J. F. de Bobadilla and A. Romano-Velazquez.
  • Video (YouTube), Video (Vimeo), Slides
• • 3 February 2022
  • Wendelin Lutz (Imperial)
  • A geometric proof of the classification of T-polygons
  • One formulation of mirror symmetry predicts (omitting a few adjectives) a one-to-one correspondence between equivalence classes of lattice polygons and deformation families of del Pezzo surfaces. Lattice polygons that correspond to smooth Del Pezzo surfaces are called T-polygons and have been classified by Kasprzyk-Nill-Prince using combinatorial methods, thereby verifying the conjecture in the smooth case. I will give a new geometric proof of their classification result.
  • Video (YouTube), Video (Vimeo), Slides
• • 27 January 2022
  • Florin Ambro (Simion Stoilow)
  • On Seshadri constants
  • The Seshadri constant of a polarized variety (X,L) at a point x measures how positive is the polarization L at x. If x is very general, the Seshadri constant does not depend on x, and captures global information on X. Inspired by ideas from the Geometry of Numbers, we introduce in this talk successive Seshadri minima, such that the first one is the Seshadri constant at a point, and the last one is the width of the polarization at the point. Assuming the point is very general, we obtain two results: a) the product of the successive Seshadri minima is proportional to the volume of the polarization; b) if X is toric, the i-th successive Seshadri constant is proportional to the i-th successive minima of a suitable 0-symmetric convex body. Based on joint work with Atsushi Ito.
  • Video (YouTube), Video (Vimeo), Slides
• • 20 January 2022
  • Marvin Hahn (Sorbonne)
  • The tropical geometry of monotone Hurwitz numbers
  • Hurwitz numbers are important enumerative invariants in algebraic geometry. They count branched maps between Riemann surfaces. Equivalently, they enumerate factorizations in the symmetric group. Hurwitz numbers were introduced in the 1890s by Adolf Hurwitz and became central objects of enumerative algebraic geometry in the 1990s through close connections with the so-called Gromov–Witten theory. This interplay between Hurwitz and Gromov–Witten theory is an active field of research and led to, among other things, the celebrated ELSV formula. In the last decade, many variants of Hurwitz numbers have been introduced and studied. In particular, the question of connections between these variants of Hurwitz numbers and Gromov–Witten theory is of great interest. So-called monotone Hurwitz numbers , which originate from the theory of random matrices, are among the most studied variants of Hurwitz numbers. This talk is a progress report of our larger program in which we study the connections between monotone Hurwitz numbers and Gromov–Witten theory by combinatorial methods of tropical geometry, and whose long-term goal is a proof of the still open conjecture of an ELSV - type formula for double monotone Hurwitz numbers. The talk is based in part on joint work with Reinier Kramer and Danilo Lewanski.
  • Video (YouTube), Video (Vimeo), Slides
• • 13 January 2022
  • Arina Voorhaar (Geneva)
  • On the Newton Polytope of the Morse Discriminant
  • A famous classical result by Gelfand, Kapranov and Zelevinsky provides a combinatorial description of the vertices of the Newton polytope of the A-discriminant (the closure of the set of all non-smooth hypersurfaces defined by polynomials with the given support A). Namely, it gives a surjection from the set of all convex triangulations of the convex hull of the set A with vertices in A (or, equivalently, the set of all possible combinatorial types of smooth tropical hypersurfaces defined by tropical polynomials with support A) onto the set of vertices of this Newton polytope. In my talk, I will discuss a similar problem for the Morse discriminant — the closure of the set of all polynomials with the given support A which are non-Morse if viewed as polynomial maps. Namely, for a 1-dimensional support set A, there is a surjection from the set of all possible combinatorial types of so-called Morse tropical polynomials onto the vertices of the Newton polytope of the Morse discriminant.
  • Video (YouTube), Video (Vimeo), Slides
• • 8 December 2021
  • Eleonore Faber (Leeds)
  • Matrix factorizations for discriminants of pseudo-reflection groups
  • In this talk we will give an introduction to the McKay correspondence for complex reflection groups (joint work with Ragnar Buchweitz and Colin Ingalls), and then show how this allows to identify certain matrix factorizations of the discriminants of these reflection groups. We will in particular consider the family of pseudo-reflection groups G(r,p,n), for which one can explicitly determine matrix factorizations, using higher Specht polynomials (work in progress with Colin Ingalls, Simon May, and Marco Talarico).
  • Video (YouTube), Video (Vimeo), Slides
• • 25 November 2021
  • Ana Ros Camacho (Cardiff)
  • Computational aspects in orbifold equivalence
  • Landau-Ginzburg models are a family of physical theories described by some polynomial (or "potential") characterized by having an isolated singularity at the origin. Often appearing in mirror-symmetric phenomena, they can be collected in higher categories with nice properties that allow direct computations. In this context, it is possible to introduce an equivalence relation between two different potentials called "orbifold equivalence". We will present some recent examples of this equivalence, and discuss the computational challenges posed by the search of new ones. Joint work with Timo Kluck.
  • Video (YouTube), Video (Vimeo), Slides
• • 18 November 2021
  • Nicholas Anderson (Queen Mary)
  • Paving tropical ideals
  • Tropical geometry is a powerful tool in algebraic geometry, which offers a multitude of combinatorial approaches to studying algebraic varieties. This talk will focus on the recent development of tropical commutative algebra by Diane Maclagan and Felipe Rincon. The central object of study is the "tropical Ideal", which generalizes the structure of polynomial ideals over fields to be suitable for study in the setting of tropical geometry, that is, in polynomial semirings over semifields. All polynomial ideals over a field can be associated to a "realizable" tropical ideal, and it is a non-trivial fact that "non-realizable" tropical ideals exist. In this talk, I will demonstrate how the combinatorics of matroid theory allows us to easily generate a subclass of tropical ideals, called paving tropical ideals, which in turn allows us to prove that most zero-dimensional tropical ideals are not realizable.
  • Video (YouTube), Video (Vimeo), Slides
• • 28 October 2021
  • Andrea Brini (Sheffield)
  • Quantum geometry of log-Calabi Yau surfaces
  • A log-Calabi Yau surface with maximal boundary, or Looijenga pair, is a pair (X,D) with X a smooth complex projective surface and D a singular anticanonical divisor in X. I will introduce a series of physics-motivated correspondences relating five different classes of enumerative invariants of the pair (X,D):
    • the log Gromov–Witten theory of (X,D),
    • the Gromov–Witten theory of X twisted by the sum of the dual line bundles to the irreducible components of D,
    • the open Gromov–Witten theory of special Lagrangians in a toric Calabi–Yau 3-fold determined by (X,D),
    • the Donaldson–Thomas theory of a symmetric quiver specified by (X,D), and
    • a class of BPS invariants considered in different contexts by Klemm–Pandharipande, Ionel–Parker, and Labastida–Marino–Ooguri–Vafa.
    I will also show how the problem of computing all these invariants is closed-form solvable. Based on joint works with P. Bousseau, M. van Garrel, and Y. Schueler.
  • Video (YouTube), Video (Vimeo), Slides
• • 21 October 2021
  • Anne-Sophie Kaloghiros (Brunel)
  • The Calabi problem for Fano 3-folds
  • I will discuss progress on the Calabi problem for Fano 3-folds. The 105 deformation families of smooth Fano 3-folds, were classified by Iskovskikh, Mori and Mukai. We determine whether or not the general member of each of these 105 families admits a Kähler-Einstein metric. In some cases, it is known that while the general member of the family admits a Kähler-Einstein metric, some other member does not. This leads to the problem of determining which members of a deformation family admit a Kähler-Einstein metric when the general member does. This is accomplished for most of the families, and I will present a conjectural picture for some of the remaining families. This is a joint project with Carolina Araujo, Ana-Maria Castravet, Ivan Cheltsov, Kento Fujita, Jesus Martinez-Garcia, Constantin Shramov, Hendrik Süss and Nivedita Viswanathan.
  • Video (YouTube), Video (Vimeo), Slides
• • 14 October 2021
  • Alastair Craw (Bath)
  • Hyperpolygon spaces: beyond the movable cone
  • For n>=4, the hyperpolygon spaces are a collection of Nakajima quiver varieties in dimension 2n-6 that have been a useful testing ground for conjectures on conical symplectic varieties. I'll describe joint work in progress with Gwyn Bellamy, Steven Rayan, Travis Schedler and Hartmut Weiss in which we describe completely the birational geometry of these spaces. The case n=5 recovers a well-known finite quotient singularity in dimension four, and allows us to provide a uniform construction of all 81 projective crepant resolutions studied in previous work of Donten-Bury–Wiśniewski. I'll also explain the title of the talk by giving a geometric interpretation of the components of the stability parameter even when it doesn't lie in the positive orthant.
  • Video (YouTube), Video (Vimeo), Slides
• • 7 October 2021
  • Tom Coates (Imperial)
  • Rigid maximally mutable Laurent polynomials
  • I will describe a class of Laurent polynomials which conjecturally corresponds under mirror symmetry to Fano varieties, in any dimension, with mild singularities. This is joint work with Alexander Kasprzyk, Giuseppe Pitton, and Ketil Tveiten.
  • Video (YouTube), Video (Vimeo), Slides
• • 30 September 2021
  • Julius Giesler (Tübingen)
  • Kanev and Todorov type surfaces in toric 3-folds
  • In this talk we show at the example of some surfaces of general type, so called Kanev and Todorov type surfaces, how to construct minimal and canonical models of hypersurfaces in toric varieties. We relate the plurigenera and the Kodaira dimension of the hypersurfaces to a special polytope, known as the Fine interior. Then we study singularities of the canonical models of Kanev/Todorov type surfaces via toric geometry, degenerations of these surfaces and investigate some Hodge theoretic consequences.
  • Video (YouTube), Video (Vimeo), Slides
• • 23 September 2021
  • Nicolas Addington (Oregon)
  • Hodge number are not derived invariants in positive characteristic
  • Derived categories of coherent sheaves behave a lot like cohomology, so it's natural to ask which cohomological invariants are preserved by derived equivalences. After discussing the motivation and previous results, I'll present a derived equivalence between Calabi–Yau 3-folds in characteristic 3 with different Hodge numbers; this couldn't happen in characteristic 0. The project has a substantial computer algebra component which I'll spend some time on.
  • Video (YouTube), Video (Vimeo), Slides
• • 16 September 2021
  • Nikita Nikolaev (Sheffield)
  • Abelianisation of Meromorphic Connections
  • There is a natural 1-1 correspondence between Higgs bundles on a compact complex curve and line bundles on an appropriate branched cover. This abelianisation process goes through the direct image functor and it has been fruitful in addressing a variety of problems relating to bundles on curves. We extend this abelianisation correspondence from Higgs bundles to flat bundles. This generalisation involves choosing a certain graph which translates to cohomology as a natural cocycle that exhibits a local deformation of the direct image functor. Furthermore, our abelianisation correspondence extends to lambda-connections and recovers the abelianisation of Higgs bundles as lambda goes to 0. Based in part on joint work in progress with Marco Gualtieri.
  • Video (YouTube), Video (Vimeo), Slides
• • 2 September 2021
  • Hunter Spink (Stanford)
  • Log-concavity of matroid h-vectors and mixed Eulerian numbers
  • For any matroid M, we compute the Tutte polynomial using the mixed intersection numbers of certain tautological classes in the combinatorial Chow ring arising from Grassmannians. Using mixed Hodge-Riemann relations, we deduce a strengthening of the log-concavity of the h-vector of a matroid complex, improving on an old conjecture of Dawson that was resolved contemporaneously by Ardila, Denham, and Huh. Joint with Andrew Berget and Dennis Tseng.
  • Video (YouTube), Video (Vimeo)
• • 26 August 2021
  • DongSeon Hwang (Ajou)
  • Cascades of singular rational surfaces of Picard number one
  • I will introduce the notion of cascades of singular rational surfaces of Picard number one, which consists of a sequence of special birational morphisms, and then discuss some applications in the toric case, Fano case, and (log) general type case. The latter application is closely related to algebraic Montgomery-Yang problem, conjectured by Kollár.
  • Video (YouTube), Video (Vimeo), Slides
• • 12 August 2021
  • Ollie Clarke (Bristol and Ghent)
  • Combinatorial mutations and block diagonal polytopes
  • Matching fields were introduced by Sturmfels and Zelevinsky to study certain Newton polytopes and more recently have been shown to give rise to toric degenerations of various families of varieties. Whenever a matching field gives rise to a toric degeneration of the Grassmannian, the polytope of the associated toric variety coincides with the matching field polytope. In this talk I will describe combinatorial mutations of matching field polytopes. We will explore properties of polytopes which are preserved by mutation, and we will see that property of giving rise to a toric degeneration is preserved by mutations. This gives us an easy way to generate new families of toric degenerations of the Grassmannian from old. This talk is based on joint work with Akihiro Higashitani and Fatemeh Mohammadi.
  • Video (YouTube), Video (Vimeo), Slides
• • 5 August 2021
  • Dhruv Ranganathan (Cambridge)
  • Toric contact cycles in the moduli space of curves
  • The toric contact cycles are loci in the moduli space of curves that parameterize those curves that admit a morphism to a fixed toric variety, with prescribed tangency data with the toric boundary. The cycles are the fundamental building blocks in higher genus logarithmic Gromov–Witten theory and are higher dimensional analogues of the double ramification cycles, which have been studied intensely in the last decade. In recent work, Sam Molcho (ETH) and I proved that these cycles lie in the tautological part of the Chow ring of the moduli space of curves. A lesson I learned from this project, and earlier work with Navid Nabijou (Cambridge), is that it can be quite profitable to blend Fulton’s analysis of blowups and strict transforms with logarithmic Gromov–Witten theory and its virtual class. I’ll try to give a sense of the basic geometric phenomena, and point to some other places where they come up.
  • Video (YouTube), Video (Vimeo), Slides
• • 29 July 2021
  • Matthias Nickel (Frankfurt)
  • Local positivity and effective Diophantine approximation
  • In this talk I will discuss a new approach to prove effective results in Diophantine approximation relying on lower bounds of Seshadri constants. I will then show how to use it to prove an effective theorem on the simultaneous approximation of two algebraic numbers satisfying an algebraic equation.
  • Video (YouTube), Video (Vimeo), Slides
• • 22 July 2021
  • Kristin DeVleming (UCSD)
  • K moduli of quartic K3 surfaces
  • We will discuss a family of compactifications of moduli spaces of log Fano pairs coming from K-stability, and discuss an application to moduli of quartic K3 surfaces, with a focus on the locus of hyperelliptic K3s that arise as double covers of P¹xP¹ branched over a (4,4) curve. We will show that K-stability provides a natural way to interpolate between the GIT moduli space and the Baily-Borel compactification and will relate this interpolation to VGIT wall crossings. This is joint work with Kenny Ascher and Yuchen Liu.
  • Video (YouTube), Video (Vimeo)
• • 15 July 2021
  • Sergey Galkin (PUC-Rio and HSE)
  • Graph potentials and combinatorial non-abelian Torelli
  • I will introduce graph potentials and discuss some of their combinatorial aspects, such as small resolution conjecture and combinatorial non-abelian Torelli theorem. The talk is based on the joint works with Pieter Belmans and Swarnava Mukhopadhyay.
  • Video (YouTube), Video (Vimeo), Slides
• • 8 July 2021
  • Chengxi Wang (UCLA)
  • Varieties of general type with small volume
  • By Hacon-McKernan, Takayama, and Tsuji, there is a constant r_n such that for every r at least r_n, the r-canonical map of every n-dimensional variety of general type is birational. In this talk, we show that r_n must grow faster than any polynomial in n, by giving examples of general type with small volume in high dimensions. In particular, we construct a klt n-fold with ample canonical class whose volume is less than 1/2²ⁿ. The klt examples should be close to optimal. This is joint work with Burt Totaro.
  • Video (YouTube), Video (Vimeo), Slides
• • 1 July 2021
  • Pedro Montero (Valparaíso)
  • On the liftability of the automorphism group of smooth hypersurfaces of the projective space
  • Smooth hypersurfaces are classical objects in algebraic geometry since they are the simplest varieties one can define as they are given by only one equation. As such, they have been intensively studied and their geometry has shaped the development of classic and modern algebraic geometry. In this talk, I will first recall some fundamental results concerning the automorphism group of smooth hypersurfaces of the projective space and then I will present some new results obtained in a joint work with Victor Gonzalez-Aguilera and Alvaro Liendo, which are inspired by the classification groups which faithfully act on smooth cubic and quintic threefolds by Oguiso, Wei and Yu. Finally, I will discuss some perspectives and open problems that arise from this.
  • Video (YouTube), Video (Vimeo), Slides
• • 24 June 2021
  • Yusuke Nakajima (Kyoto)
  • Combinatorial mutations and deformations of dimer models
  • The combinatorial mutation of a polytope was introduced in the context of the mirror symmetry of Fano manifolds for achieving the classification problem. This operation makes a given polytope another one while keeping some properties. In my talk, I will consider the combinatorial mutation of a polygon associated to a dimer model. A dimer model is a bipartite graph on the real two-torus, and the combinatorics of a dimer model gives rise to a certain lattice polygon. Also, a dimer model enjoys rich information regarding toric geometry associated to that polygon. It is known that for any lattice polygon P there is a dimer model whose associated polygon coincides with P. Thus, there also exists a dimer model giving the lattice polygon obtained as the combinatorial mutation of P. I will observe the relationship between a dimer model giving a lattice polygon P and the one giving the combinatorial mutation of P. In particular, I introduce the operation which I call the deformation of a dimer model, and show that this operation induces the combinatorial mutation of a polygon associated to a dimer model. This talk is based on a joint work with A. Higashitani.
  • Video (YouTube), Video (Vimeo), Slides
• • 17 June 2021
  • Laura Escobar (Washington)
  • Wall-crossing phenomenon for Newton-Okounkov bodies
  • A Newton-Okounkov body is a convex set associated to a projective variety, equipped with a valuation. These bodies generalize the theory of Newton polytopes and the correspondence between polytopes and projective toric varieties. Work of Kaveh-Manon gives an explicit link between tropical geometry and Newton-Okounkov bodies. We use this link to describe a wall-crossing phenomenon for Newton-Okounkov bodies. As an example, we describe wall-crossing formula in the case of the Grassmannian Gr(2,m). This is joint work with Megumi Harada.
  • Video (YouTube), Video (Vimeo), Slides
• • 10 June 2021
  • Anne Lonjou (Paris-Saclay)
  • Action of Cremona groups on CAT(0) cube complexes
  • A key tool to study the plane Cremona group is its action on a hyperbolic space. Sadly, in higher rank such an action is not available. Recently, in geometric group theory, actions on CAT(0) cube complexes turned out to be a powerful tool to study a large class of groups. In this talk, based on a common work with Christian Urech, we will construct such complexes on which Cremona groups of rank n act. Then, we will see which kind of results on these groups we can obtain.
  • Video (YouTube), Video (Vimeo), Slides
• • 3 June 2021
  • Martin Ulirsch (Frankfurt)
  • Parabolic Higgs bundles on toric varieties
  • In this talk I will explain a version of Simpson’s non-abelian Hodge correspondence on a toric variety X. There is a natural 1-1 correspondence between stable parabolic Higgs bundles on X and irreducible representations of the fundamental group of the big torus. This correspondence reduces to a correspondence between toric vector bundles and integral unitary representations in a suitable sense. In this story the spherical Tits building will have a surprise appearance. The main result suggests (at least to me) that there is a yet-to-be-discovered logarithmic incarnation of the non-abelian Hodge correspondence.
  • Video (YouTube), Video (Vimeo), Slides
• • 27 May 2021
  • Helge Ruddat (Mainz)
  • Polytopes, periods, degenerations
  • A lattice polytope describes a projective toric variety and a regular subdivision of the polytope describes a flat degeneration of the toric variety. It is instructive to deform the degenerating family in a way that makes the geometry non-toric and produces a more interesting real torus fibration on the fibres of the degeneration. I am going to explain a simple formula that permits the easy computation of period integrals for the deformed families. This approach to periods doesn't require any differential equations and is flexible enough to give proofs for strong results about Gross-Siebert's degenerating families obtained from wall structures. The talk is based on joint work with Bernd Siebert.
  • Video (YouTube), Video (Vimeo), Slides
• • 20 May 2021
  • Hannah Markwig (Tübingen)
  • Counting bitangents of plane quartics - tropical, real and arithmetic
  • A smooth plane quartic defined over the complex numbers has precisely 28 bitangents. This result goes back to Pluecker. In the tropical world, the situation is different. One can define equivalence classes of tropical bitangents of which there are 7, and each has 4 lifts over the complex numbers. Over the reals, we can have 4, 8, 16 or 28 bitangents. The avoidance locus of a real quartic is the set in the dual plane consisting of all lines which do not meet the quartic. Every connected component of the avoidance locus has precisely 4 bitangents in its closure. For any field k of characteristic not equal to 2 and with a non-Archimedean valuation which allows us to tropicalize, we show that a tropical bitangent class of a quartic either has 0 or 4 lifts over k. This way of grouping into sets of 4 which exists tropically and over the reals is intimately connected: roughly, tropical bitangent classes can be viewed as tropicalizations of closures of connected components of the avoidance locus. Arithmetic counts offer a bridge connecting real and complex counts, and we investigate how tropical geometry can be used to study this bridge. This talk is based on joint work with Maria Angelica Cueto, and on joint work in progress with Sam Payne and Kristin Shaw.
  • Video (YouTube), Video (Vimeo), Slides
• • 13 May 2021
  • Roger Casals (UC Davis)
  • Positroid links and braid varieties
  • I will discuss a class of affine algebraic varieties associated to positive braids, their relation to open positroid strata in Grassmannians and their cluster structures. First, I will introduce the objects of interest, with the necessary ingredients, and motivate the problem at hand. Then we will discuss in detail how the study of a DG-algebra associated to certain links may allow us to better understand the algebraic (and cluster) geometry of Richardson and positroid varieties. Explicit examples of this interplay between topology and algebraic geometry will be illustrated. At a more conceptual level, the talk brings to bear insight from symplectic topology to better understand positroid varieties. This is joint work with E. Gorsky, M. Gorsky and J. Simental.
  • Video (YouTube), Video (Vimeo), Slides
• • 5 May 2021
  • Travis Mandel (Oklahoma)
  • Quantum theta bases for quantum cluster algebras
  • One of the central goals in the study of cluster algebras is to better understand various canonical bases and positivity properties of the cluster algebras and their quantizations. Gross-Hacking-Keel-Kontsevich (GHKK) applied ideas from mirror symmetry to construct so-called "theta bases" for cluster algebras which satisfy all the desired positivity properties, thus proving several conjectures regarding cluster algebras. I will discuss joint work with Ben Davison in which we combine the techniques used by GHKK with ideas from the DT theory of quiver representations to quantize the GHKK construction, thus producing quantum theta bases and proving the desired quantum positivity properties.
  • Video (YouTube), Video (Vimeo), Slides
• • 29 April 2021
  • Michael Wemyss (Glasgow)
  • Jacobi algebras on the two-loop quiver and applications
  • I will explain recent progress on classifying finite dimensional Jacobi algebras on the two loop quiver. This is a purely algebraic problem, which at first sight is both seemingly hopeless and seemingly detached from any form of reality or wider motivation. There are two surprises: first, the problem is not hopeless, and parts of the answer are in fact very beautiful. Second, this has immediate and surprising consequences to both 3-fold flops and 3-fold divisor-to-curve contractions, their curve invariants and their conjectural classification. This is joint work with Gavin Brown.
  • Video (YouTube), Video (Vimeo), Slides
• • 22 April 2021
  • Ben Wormleighton (Washington)
  • A tale of two widths: lattice and Gromov
  • To a polytope P whose facet normals are rational one can associate two geometric objects: a symplectic toric domain X_P and a polarised toric algebraic variety Y_P, which can also be viewed as a potentially singular symplectic space. A basic invariant of a symplectic manifold X is its Gromov width: essentially the size of the largest ball that can be 'symplectically' embedded in X. A conjecture of Averkov-Hofscheier-Nill proposed a combinatorial bound for the Gromov width of Y_P, which I recently verified in dimension two with Julian Chaidez. I’ll discuss the proof, which goes via various symplectic and algebraic invariants with winsome combinatorial interpretations in the toric case. If there’s time, I’ll discuss ongoing work and new challenges for a similar result in higher dimensions.
  • Video (YouTube), Video (Vimeo), Slides
• • 15 April 2021
  • Johannes Nordström (Bath)
  • Extra-twisted connected sum G₂-manifolds
  • The twisted connected sum construction of Kovalev produces many examples of closed Riemannian 7-manifolds with holonomy group G₂ (a special class of Ricci-flat manifolds), starting from complex algebraic geometry data like Fano 3-folds. If the pieces admit automorphisms, then adding an extra twist to the construction yields examples with a wider variety of topological features. I will describe the constructions and outline how one can use them to produce example of e.g. closed 7-manifolds with disconnected moduli space of holonomy G₂ metrics, or pairs of G₂-manifolds that homeomorphic but not diffeomorphic. This is joint work with Diarmuid Crowley and Sebastian Goette.
  • Video (YouTube), Video (Vimeo), Slides
• • 8 April 2021
  • Tim Gräfnitz (Hamburg)
  • Tropical correspondence for smooth del Pezzo log Calabi–Yau pairs
  • In this talk I will present the main results of my thesis, a tropical correspondence theorem for log Calabi–Yau pairs (X,D) consisting of a smooth del Pezzo surface X of degree >=3 and a smooth anticanonical divisor D. The easiest example of such a pair is (P²,E), where E is an elliptic curve. I will explain how the genus zero logarithmic Gromov–Witten invariants of X with maximal tangency to D are related to tropical curves in the dual intersection complex of (X,D) and how they can be read off from the consistent wall structure appearing in the Gross-Siebert program. The novelty in this correspondence is that D is smooth but non-toric, leading to log singularities in the toric degeneration that have to be resolved.
  • Video (YouTube), Video (Vimeo), Slides
• • 1 April 2021
  • JiaRui Fei (Shanghai Jiao Tong)
  • Tropical F-polynomials and Cluster Algebras
  • The representation-theoretic interpretations of g-vectors and F-polynomials are two fundamental ingredients in the (additive) categorification of cluster algebras. We knew that the g-vectors are related to the presentation spaces. We introduce the tropical F-polynomial f_M of a quiver representation M, and explain its interplay with the general presentation for any finite-dimensional basic algebra. As a consequence, we give a presentation of the Newton polytope N(M) of M. We propose an algorithm to determine the generic Newton polytopes, and show it works for path algebras. As an application, we give a representation-theoretic interpretation of Fock-Goncharov's cluster duality pairing. We also study many combinatorial aspects of N(M), such as faces, the dual fan and 1-skeleton. We conjecture that the coefficients of a cluster monomial corresponding to vertices are all 1, and the coefficients inside the Newton polytope are saturated. We show the conjecture holds for acyclic cluster algebras. We specialize the above general results to the cluster-finite algebras and the preprojective algebras of Dynkin type.
  • Video (YouTube), Video (Vimeo), Slides
• • 25 March 2021
  • Michał Kapustka (IMPAN and Stavanger)
  • Nikulin orbifolds
  • The theory of K3 surfaces with symplectic involutions and their quotients is now a well understood classicial subject thanks to foundational works of Nikulin, and van Geemen and Sarti. In this talk we will try to develop an analogous theory in the context of hyperkahler fourfolds of K3^[2] type. First, we will present a latttice theoretic classification of such fourfolds which admit a symplectic involution. Then we will investigate the associated quotients that we call Nikulin orbifolds. These are orbifolds which admit a symplectic form on the smooth locus and hence are special cases of so called hyperkahler orbifolds. Finally, we shall discuss families of Nikulin orbifolds and their deformations called hyperkahler orbifolds of Nikulin type. As an application, we will provide a description of the first known example of a complete family of projective hyperkahler orbifolds. This is joint work with A. Garbagnati, C. Camere and G. Kapustka.
  • Video (YouTube), Video (Vimeo), Slides
• • 18 March 2021
  • Taro Sano (Kobe)
  • Construction of non-Kähler Calabi–Yau manifolds by log deformations
  • Calabi–Yau manifolds (in the strict sense) form an important class in the classification of algebraic varieties. One can also consider its generalisation by removing the projectivity assumption. It was previously known that there are infinitely many topological types of non-Kähler Calabi–Yau 3-folds. In this talk, I will present construction of such examples in higher dimensions by smoothing normal crossing varieties. The key tools of the construction are some isomorphisms between general rational elliptic surfaces which induce isomorphisms between Calabi–Yau manifolds of Schoen type.
  • Video (YouTube), Video (Vimeo), Slides
• • 11 March 2021
  • Diane Maclagan (Warwick)
  • Toric and tropical Bertini theorems in arbitrary characteristic
  • The classical Bertini theorem on irreducibility when intersecting by hyperplanes is a standard part of the algebraic geometry toolkit. This was generalised recently, in characteristic zero, by Fuchs, Mantova, and Zannier to a toric Bertini theorem for subvarieties of an algebraic torus, with hyperplanes replaced by subtori. I will discuss joint work with Gandini, Hering, Mohammadi, Rajchgot, Wheeler, and Yu in which we give a different proof of this theorem that removes the characteristic assumption. An application is a tropical Bertini theorem.
  • Video (YouTube), Video (Vimeo), Slides
• • 4 March 2021
  • Fatemeh Rezaee (Loughborough)
  • Wall-crossing does not induce MMP
  • I will describe a new wall-crossing phenomenon of sheaves on the projective 3-space that induces singularities that are not allowed in the sense of the Minimal Model Program (MMP). Therefore, it cannot be detected as an operation in the MMP of the moduli space, unlike the case for many surfaces.
  • Slides
• • 23-26 February 2021
  • Fano Varieties and Birational Geometry
  • An online workshop exploring recent developments in the geometry of Fano varieties.
  • Workshop webpage
• • 18 February 2021
  • Francesco Zucconi (Udine)
  • Fujita decomposition and Massey product for fibered varieties
  • Let f:X -> B be a semistable fibration where X is a smooth variety of dimension n >= 2 and B is a smooth curve. We give an interpretation of the second Fujita decomposition of f_*ωX/B in terms of local systems of the relative 1-forms and of the relative top forms. We show the existence of higher irrational pencils under natural hypothesis on local subsystems.
  • Video (YouTube), Video (Vimeo), Slides
• • 11 February 2021
  • Enrica Mazzon (Bonn)
  • Non-archimedean approach to mirror symmetry and to degenerations of varieties
  • Mirror symmetry is a fast-moving research area at the boundary between mathematics and theoretical physics. Originated from observations in string theory, it suggests that complex Calabi–Yau manifolds should come in mirror pairs, in the sense that geometrical information of a Calabi–Yau manifold can be read through invariants of its mirror. In the first part of the talk, I will introduce some geometrical ideas inspired by mirror symmetry. In particular, I will go through the main steps which relate mirror symmetry to non-archimedean geometry and the theory of Berkovich spaces. In the second part, I will describe a combinatorial object, the so-called dual complex of a degeneration of varieties. This emerges in many contexts of algebraic geometry, including mirror symmetry where moreover it comes equipped with an integral affine structure. I will show how the techniques of Berkovich geometry give a new insight into the study of dual complexes and their integral affine structure. This is based on a joint work with Morgan Brown and a work in progress with Léonard Pille-Schneider.
  • Video (YouTube), Video (Vimeo), Slides
• • 4 February 2021
  • Pieter Belmans (Bonn)
  • Hochschild cohomology of partial flag varieties and Fano 3-folds
  • The Hochschild-Kostant-Rosenberg decomposition gives a description of the Hochschild cohomology of a smooth projective variety in terms of the sheaf cohomology of exterior powers of the tangent bundle. In all but a few cases it is a non-trivial task to compute this decomposition, and understand the extra algebraic structure which exists on Hochschild cohomology. I will give a general introduction to Hochschild cohomology and this decomposition, and explain what it looks like for partial flag varieties (joint work with Maxim Smirnov) and Fano 3-folds (joint work with Enrico Fatighenti and Fabio Tanturri).
  • Video (YouTube), Video (Vimeo), Slides
• • 28 January 2021
  • Matej Filip (Ljubljana)
  • The miniversal deformation of an affine toric Gorenstein threefold
  • We are going to describe the reduced miniversal deformation of an affine toric Gorenstein threefold. The reduced deformation components correspond to special Laurent polynomials. There is canonical bijective map between the set of the smoothing components and the set of the corresponding Laurent polynomials, which we are going to analyse in more details.
  • Video (YouTube), Video (Vimeo)
• • 21 January 2021
  • Michel van Garrel (Birmingham)
  • Stable maps to Looijenga pairs
  • Start with a rational surface Y admitting a decomposition of its anticanonical divisor into at least two smooth nef components. We associate five curve counting theories to this Looijenga pair: 1) all genus stable log maps with maximal tangency to each boundary component; 2) genus zero stable maps to the local Calabi–Yau surface obtained by twisting Y by the sum of the line bundles dual to the components of the boundary; 3) the all genus open Gromov–Witten theory of a toric Calabi–Yau threefold associated to the Looijenga pair; 4) the Donaldson-Thomas theory of a symmetric quiver specified by the Looijenga pair and 5) BPS invariants associated to the various curve counting theories. In this joint work with Pierrick Bousseau and Andrea Brini, we provide closed-form solutions to essentially all of the associated invariants and show that the theories are equivalent. I will start by describing the geometric transitions from one geometry to the other, then give an overview of the curve counting theories and their relations. I will end by describing how the scattering diagrams of Gross and Siebert are a natural place to count stable log maps.
  • Video (YouTube), Video (Vimeo), Slides
• • 14 January 2021
  • Lawrence Barrott (Boston College)
  • Log geometry and Chow theory
  • Log geometry has become a central tool in enumerative geometry over the past years, providing means to study many degenerations situations. Unfortunately much of the theory is complicated by the fact that products of log schemes differ from products of schemes. In this talk I will introduce a gadget which replaces Chow theory for log schemes, reproducing many familiar tools such as virtual pullback in the context of log geometry.
  • Video (YouTube), Video (Vimeo), Slides
• • 10 December 2020
  • Chris Eur (Stanford)
  • Tautological bundles of matroids
  • Recent advances in matroid theory via tropical geometry broadly fall into two themes: One concerns the K-theory of Grassmannians, and the other concerns the intersection theory of wonderful compactifications. How do these two themes talk to each other? We introduce the notion of tautological bundles of matroids to unite these two themes. As a result, we give a geometric interpretation of the Tutte polynomial of a matroid that unifies several previous works as its corollaries, deduce new log-concavity statements, and answer few conjectures in the literature. This is an ongoing project with Andrew Berget, Hunter Spink, and Dennis Tseng.
  • Slides
• • 3 December 2020
  • Tim Magee (Birmingham)
  • Convexity in tropical spaces and compactifications of cluster varieties
  • Cluster varieties are a relatively new, broadly interesting class of geometric objects that generalise toric varieties. Convexity is a key notion in toric geometry. For instance, projective toric varieties are defined by convex lattice polytopes. In this talk, I'll explain how convexity generalises to the cluster world, where "polytopes" live in a tropical space rather than a vector space and "convex polytopes" define projective compactifications of cluster varieties. Time permitting, I'll conclude with two exciting applications of this more general notion of convexity: 1) an intrinsic version of Newton-Okounkov bodies and 2) a possible cluster version of a classic toric mirror symmetry construction due to Batyrev. Based on joint work with Man-Wai Cheung and Alfredo Nájera Chávez.
  • Video (YouTube), Video (Vimeo), Slides
• • 26 November 2020
  • Okke van Garderen (Glasgow)
  • Refined Donaldson-Thomas theory of threefold flops
  • DT invariants are virtual counts of semistable objects in the derived category of a Calabi–Yau variety, which can be calculated at various levels of refinement. An interesting family of CY variety which are of particular interest to the MMP are threefold flopping curves, and in this talk I will explain how to understand their DT theory. The key point is that the stability conditions on the derived categories can be understood via tilting equivalences, which can be seen as the analogue of cluster mutations in this setting. I will show that these equivalences induce wall-crossing formulas, and use this to reduce the DT theory of a flop to a comprehensible set of curve-counting invariants, which can be computed for several examples. These computations produce new evidence for a conjecture of Pandharipande-Thomas, and show that refined DT invariants are not enough to completely classify flops.
  • Video (YouTube), Video (Vimeo), Slides
• • 20 November 2020
  • Ana Peón-Nieto (Birmingham/Côte d'Azur)
  • Pure codimensionality of wobbly bundles
  • Higgs bundles on smooth projective curves were introduced by Hitchin as solutions to gauge equations motivated by physics. They can be seen as points of T*N, where N is the moduli space of vector bundles on the curve. The topology of the moduli space of Higgs bundles is determined by the nilpotent cone, which is a reducible scheme containing the zero section of T*N--->N. Inside this section, wobbly bundles are particularly important, as this is the locus where any other component intersects N. In fact, this implies that the geometry of the nilpotent cone can be described in terms of wobbly bundles. In this talk I will explain an inductive method to prove pure codimensionality of the wobbly locus, as announced in a paper by Laumon from the 80's. We expect our method to yield moreover a description of the irreducible components of the nilpotent cone in arbitrary rank.
  • Slides
• • 19 November 2020
  • Naoki Fujita (Tokyo)
  • Newton-Okounkov bodies arising from cluster structures
  • A toric degeneration is a flat degeneration from a projective variety to a toric variety, which can be used to apply the theory of toric varieties to other projective varieties. In this talk, we discuss relations among the following three constructions of toric degenerations: representation theory, Newton-Okounkov bodies, and cluster algebras. More precisely, we construct Newton-Okounkov bodies using cluster structures, and realize representation-theoretic and cluster-theoretic toric degenerations from this framework. As an application, we connect two kinds of representation-theoretic polytopes (string polytopes and Nakashima-Zelevinsky polytopes) by tropicalized cluster mutations. We also discuss relations with combinatorial mutations which was introduced in the context of mirror symmetry for Fano varieties. More precisely, we relate dual polytopes of these representation-theoretic polytopes by combinatorial mutations. This talk is based on joint works with Hironori Oya and Akihiro Higashitani.
  • Video (YouTube), Video (Vimeo), Slides
• • 12 November 2020
  • Arkadij Bojko (Oxford)
  • Orientations for DT invariants on quasi-projective Calabi–Yau 4-folds
  • Donaldson-Thomas type invariants in complex dimension 4 have attracted a lot of attention in the past few years. I will give a brief overview of how one can count coherent sheaves on Calabi–Yau 4-folds. Inherent to the definition of DT4 invariants is the notion of orientations on moduli spaces of sheaves/perfect complexes. For virtual fundamental classes and virtual structure sheaves to be well-defined, one needs to prove orientability. The result of Cao-Gross-Joyce does this for projective CY 4-folds. However, computations are more feasible in the non-compact setting using localization formulae, where the fixed point loci inherit orientations from global ones, and orientations of the virtual normal bundles come into play. I will explain how to use real determinant line bundles of Dirac operators on the double of the original Calabi–Yau manifold to construct orientations on the moduli stack of compactly supported perfect complexes, moduli schemes of stable pairs and Hilbert schemes. These are controlled by choices of orientations in K-theory and satisfy compatibility under direct sums. If time allows, I will discuss the connection between the sings obtained from comparing orientations and universal wall-crossing formulae of Joyce using vertex algebras.
  • Video (YouTube), Video (Vimeo), Slides
• • 11 November 2020
  • Enrico Fatighenti (Toulouse)
  • Fano varieties from homogeneous vector bundles
  • The idea of classifying Fano varieties using homogeneous vector bundles was behind Mukai's classification of prime Fano 3-folds. In this talk, we give a survey of some recent progress along the same lines, including a biregular rework of the non-prime Mori-Mukai 3-folds classification and some examples of higher-dimensional Fano varieties with special Hodge-theoretical properties.
  • Video (YouTube), Video (Vimeo), Slides
• • 5 November 2020
  • Federico Barbacovi (UCL)
  • Understanding the flop-flop autoequivalence using spherical functors
  • The homological interpretation of the Minimal Model Program conjectures that flips should correspond to embeddings of derived categories, and flops to equivalences. Even if the conjecture doesn't provide us with a preferred functor, there is an obvious choice: the pull-push via the fibre product. When this approach work, we obtain an interesting autoequivalence of either side of the flop, known as the "flop-flop autoequivalence". Understanding the structure of this functor (e.g. does it split as the composition of simpler functors?) is an interesting problem, and it has been extensively studied. In this talk I will explain that there is a natural, i.e. arising from the geometry, way to realise the "flop-flop autoequivalence" as the inverse of a spherical twist, and that this presentation can help us shed light on the structure of the autoequivalence itself.
  • Video (YouTube), Video (Vimeo), Slides
• • 29 October 2020
  • Catherine Cannizzo (Simons Center)
  • Towards global homological mirror symmetry for genus 2 curves
  • The first part of the talk will discuss work in arXiv:1908.04227 [math.SG] on constructing a Donaldson-Fukaya-Seidel type category for the generalized SYZ mirror of a genus 2 curve. We will explain the categorical mirror correspondence on the cohomological level. The key idea uses that a 4-torus is SYZ mirror to a 4-torus. So if we view the complex genus 2 curve as a hypersurface of a 4-torus V, a mirror can be constructed as a symplectic fibration with fiber given by the dual 4-torus V^. Hence on categories, line bundles on V are restricted to the genus 2 curve while fiber Lagrangians of V^ are parallel transported over U-shapes in the base of the mirror. Next we describe ongoing work with H. Azam, H. Lee, and C-C. M. Liu on extending the result to a global statement, namely allowing the complex and symplectic structures to vary in their real six-dimensional families. The mirror statement for this more general result relies on work of A. Kanazawa and S-C. Lau.
  • Video (YouTube), Video (Vimeo), Slides
• • 22 October 2020
  • Tyler Kelly (Birmingham)
  • What is an exoflop?
  • Aspinwall stated in 2014 that an exoflop "occurs in the gauged linear sigma-model by varying the Kahler form so that a subspace appears to shrink to a point and then reemerge 'outside' the original manifold." This description may be intangible at first for us to sink our hands into but it turns out to be a great concrete technique that relates to many things we care about as algebraic geometers! We will interpret it in this talk. I will explain in toric geometry concretely what this means for us. Afterwards, I will explain why it’s yet another reason we should listen to our string theoretic friends. Namely, I hope to have enough time to explain how it gives us applications in mirror symmetry and derived categories. Exoflops are a recurring character in my joint work with David Favero (Alberta), Chuck Doran (Alberta), and Dan Kaplan (Birmingham).
  • Video (YouTube), Video (Vimeo)
• • 15 October 2020
  • Daniel Kaplan (Birmingham)
  • Exceptional collections for invertible polynomials using VGIT
  • A sum of n monomials in n variables is said to be invertible if it is quasi-homogeneous and quasi-smooth (i.e. it has a unique singularity at the origin). To an invertible polynomial w, one can associate a maximal symmetry group, and consider the derived category of equivariant matrix factorizations of w. Joint with David Favero and Tyler Kelly, we prove this category has a full exceptional collection, using a variation of GIT result of Ballard—Favero—Katzarkov. Our proof additionally utilizes the Kreuzer-Skarke classification of invertible polynomials as Thom—Sebastiani sums of Fermat, chain, and loop polynomials. I’ll present a friendly, example-oriented illustration of our approach, review related literature, and discuss applications to mirror symmetry.
  • Video (YouTube), Video (Vimeo), Slides
• • 7 October 2020
  • Hiroshi Iritani (Kyoto)
  • Quantum cohomology of blow-ups: a conjecture
  • In this talk, I discuss a conjecture that a semiorthogonal decomposition of topological K-groups (or derived categories) due to Orlov should induce a relationship between quantum cohomology under blowups. The relationship between quantum cohomology can be described in terms of solutions to a Riemann-Hilbert problem.
  • Slides, Handout
• • 1 October 2020
  • Maxim Smirnov (Augsburg)
  • Residual categories of Grassmannians
  • Exceptional collections in derived categories of coherent sheaves have a long history going back to the pioneering work of A. Beilinson. After recalling the general setup, I will concentrate on some recent developments inspired by the homological mirror symmetry. Namely, I will define residual categories of Lefschetz decompositions and discuss a conjectural relation between the structure of quantum cohomology and residual categories. I will illustrate this relationship in the case of some isotropic Grassmannians. This is a joint work with Alexander Kuznetsov.
  • Video (YouTube), Video (Vimeo), Slides
• • 24 September 2020
  • Navid Nabijou (Cambridge)
  • Degenerating tangent curves
  • It is well-known that a smooth plane cubic E supports 9 flex lines. In higher degrees we may ask an analogous question: "How many degree d curves intersect E in a single point?" The problem of calculating such numbers has fascinated enumerative geometers for decades. Despite being an extremely classical and concrete problem, it was not until the advent of Gromov–Witten invariants in the 1990s that a general method was discovered. The resulting theory is incredibly rich, and the curve counts satisfy a suite of remarkable properties, some proven and some still conjectural. In this talk I will discuss joint work with Lawrence Barrott, in which we study the behaviour of these tangent curves as the cubic E degenerates to a cycle of lines. Using the machinery of logarithmic Gromov–Witten theory, we obtain detailed information concerning how the tangent curves degenerate along with E. The theorems we obtain are purely classical, with no reference to Gromov–Witten theory, but they do not appear to admit a classical proof. No prior knowledge of Gromov–Witten theory will be assumed.
  • Video, Slides
• • 17 September 2020
  • Ronan Terpereau (Bourgogne)
  • Actions of connected algebraic groups on rational 3-dimensional Mori fibrations
  • In this talk we will study the connected algebraic groups acting on Mori fibrations X -> Y with X a rational threefold and Y a curve or a surface. We will see how these groups can be classified, using the minimal model program (MMP) and the Sarkisov program, and how our results make possible to recover most of the classification of the connected algebraic subgroups of the Cremona group Bir(P³) obtained by Hiroshi Umemura in the 1980's when the base field is the field of complex numbers.
  • Video (YouTube), Video (Vimeo), Slides
• • 10 September 2020
  • Lara Bossinger (Oaxaca)
  • Families of Gröbner degenerations, Grassmannians, and universal cluster algebras
  • Let V be the weighted projective variety defined by a weighted homogeneous ideal J and C a maximal cone in the Gröbner fan of J with m rays. We construct a flat family over affine m-space that assembles the Gröbner degenerations of V associated with all faces of C. This is a multi-parameter generalization of the classical one-parameter Gröbner degeneration associated to a weight. We show that our family can be constructed from Kaveh-Manon's recent work on the classification of toric flat families over toric varieties: it is the pullback of a toric family defined by a Rees algebra with base X_C (the toric variety associated to C) along the universal torsor A^m -> X_C. If time permits I will explain how to apply this construction to the Grassmannians Gr(2,n) (with Plücker embedding) and Gr(3,6) (with "cluster embedding"). In each case there exists a unique maximal Gröbner cone whose associated initial ideal is the Stanley-Reisner ideal of the cluster complex. We show that the corresponding cluster algebra with universal coefficients arises as the algebra defining the flat family associated to this cone. Further, for Gr(2,n) we show how Escobar-Harada's mutation of Newton-Okounkov bodies can be recovered as tropicalized cluster mutation. This is joint work with Fatemeh Mohammadi and Alfredo Nájera Chávez.
  • Video, Slides
• • 4 September 2020
  • Andrew Harder (Lehigh)
  • Log symplectic pairs and mixed Hodge structures
  • A log symplectic pair is a pair (X,Y) consisting of a smooth projective variety X and a divisor Y in X so that there is a non-degenerate log 2-form on X with poles along Y. I will discuss the relationship between log symplectic pairs and degenerations of hyperkaehler varieties, and how this naturally leads to a class of log symplectic pairs called log symplectic pairs of "pure weight". I will discuss results which show that the classification of log symplectic pairs of pure weight is analogous to the classification of log Calabi–Yau surfaces. Time permitting, I'll discuss two classes of log symplectic pairs which are related to real hyperplane arrangements and which admit cluster type structures.
  • Video, Slides
• • 3 September 2020
  • Renato Vianna (Rio de Janeiro)
  • Sharp ellipsoid embeddings and almost-toric mutations
  • We will show how to construct volume filling ellipsoid embeddings in some 4-dimensional toric domain using mutation of almost toric compactification of those. In particular we recover the results of McDuff-Schlenk for the ball, Fenkel-Müller for product of symplectic disks and Cristofaro-Gardiner for E(2,3), giving a more explicit geometric perspective for these results. To be able to represent certain divisors, we develop the idea of symplectic tropical curves in almost toric fibrations, inspired by Mikhalkin's work for tropical curves. This is joint work with Roger Casals.
  • Video, Slides
• • 27 August 2020
  • Andrea Petracci (FU Berlin)
  • K-moduli stacks and K-moduli spaces are singular
  • Only recently a separated moduli space for (some) Fano varieties has been constructed by several algebraic geometers: this is the K-moduli stack which parametrises K-semistable Fano varieties and has a separated good moduli space. A natural question is: are these stacks and spaces smooth? This question makes sense because deformations of smooth Fano varieties are unobstructed, so moduli stacks of smooth Fano varieties are smooth. In this talk I will explain how to use toric geometry to construct examples of non-smooth points in the K-moduli stack and the K-moduli space of Fano 3-folds. This is joint work with Anne-Sophie Kaloghiros.
  • Video, Slides
• • 20 August 2020
  • Man-Wai "Mandy" Cheung (Harvard)
  • Polytopes, wall crossings, and cluster varieties
  • Cluster varieties are log Calabi–Yau varieties which are a union of algebraic tori glued by birational "mutation" maps. Partial compactifications of the varieties, studied by Gross, Hacking, Keel, and Kontsevich, generalize the polytope construction of toric varieties. However, it is not clear from the definitions how to characterize the polytopes giving compactifications of cluster varieties. We will show how to describe the compactifications easily by broken line convexity. As an application, we will see the non-integral vertex in the Newton Okounkov body of Gr(3,6) comes from broken line convexity. Further, we will also see certain positive polytopes will give us hints about the Batyrev mirror in the cluster setting. The mutations of the polytopes will be related to the almost toric fibration from the symplectic point of view. Finally, we can see how to extend the idea of gluing of tori in Floer theory which then ended up with the Family Floer Mirror for the del Pezzo surfaces of degree 5 and 6. The talk will be based on a series of joint works with Bossinger, Lin, Magee, Najera-Chavez, and Vienna.
  • Video, Slides
• • 13 August 2020
  • Nathan Ilten (Simon Fraser)
  • Type D associahedra are unobstructed
  • Generalised associahedra associated to any root system were introduced by Fomin and Zelevinsky in their study of cluster algebras. For type A root systems, one recovers the classical associahedron parametrizing triangulations of a regular n-gon. For type D root systems, one obtains a polytope parametrizing centrally symmetric triangulations of a 2n-gon. In previous work, Jan Christophersen and I showed that the Stanley-Reisner ring of the simplicial complex dual to the boundary of the classical associahedron is unobstructed, that is, has vanishing second cotangent cohomology. This could be used to find toric degenerations of the Grassmannian G(2,n). In this talk, I will describe work-in-progress that generalizes this unobstructedness result to the type D associahedron.
  • Video, Slides
• • 6 August 2020
  • Yang-Hui He (City and Oxford)
  • Universes as big data: superstrings, Calabi–Yau manifolds and machine-learning
  • We review how historically the problem of string phenomenology lead theoretical physics first to algebraic/diffenretial geometry, and then to computational geometry, and now to data science and AI. With the concrete playground of the Calabi–Yau landscape, accumulated by the collaboration of physicists, mathematicians and computer scientists over the last 4 decades, we show how the latest techniques in machine-learning can help explore problems of physical and mathematical interest.
  • Video, Slides
• • 30 July 2020
  • Benjamin Braun (Kentucky)
  • The integer decomposition property and Ehrhart unimodality for weighted projective space simplices
  • We consider lattice simplices corresponding to weighted projective spaces where one of the weights is 1. We study the integer decomposition property and Ehrhart unimodality for such simplices by focusing on restrictions regarding the multiplicity of each weight. We introduce a necessary condition for when a simplex satisfies the integer decomposition property, and classify those simplices that satisfy it in the case where there are no more than three distinct weights. We also introduce the notion of reflexive stabilizations of a simpex of this type, and show that higher-order reflexive stabilizations fail to be Ehrhart unimodal and fail to have the integer decomposition property. This is joint work with Robert Davis, Morgan Lane, and Liam Solus.
  • Video, Slides
• • 24 July 2020
  • Elana Kalashnikov (Harvard)
  • Constructing Laurent polynomial mirrors for quiver flag zero loci
  • All smooth Fano varieties of dimension at most three can be constructed as either toric complete intersections (subvarieties of toric varieties) or quiver flag zero loci (subvarieties of quiver flag varieties). Conjecturally, Fano varieties are expected to mirror certain Laurent polynomials. The construction of mirrors of Fano toric complete intersections is well-understood. In this talk, I'll discuss evidence for this conjecture by proposing a method of constructing mirrors for Fano quiver flag zero loci. A key step of the construction is via finding toric degenerations of the ambient quiver flag varieties. These degenerations generalise the Gelfand-Cetlin degeneration of flag varieties, which in the Grassmannian case has an important role in the cluster structure of its coordinate ring.
  • Video, Slides
• • 16 July 2020
  • Hendrik Süß (Manchester)
  • Normalised volumes of singularities
  • The notion of the normalised volume of a singularity has been introduced relatively recently, but plays a crucial role in the context of Einstein metrics and K-stability. After introducing this invariant my plan is to specialise quickly to the case of toric singularities and show that even in this relatively simple setting interesting phenomena occur.
  • Video, Slides
• • 15 July 2020
  • Chris Lazda (Warwick)
  • A Neron-Ogg-Shafarevich criterion for K3 surfaces
  • The naive analogue of the Néron-Ogg-Shafarevich criterion fails for K3 surfaces, that is, there exist K3 surfaces over Henselian, discretely valued fields K, with unramified etale cohomology groups, but which do not admit good reduction over K. Assuming potential semi-stable reduction, I will show how to correct this by proving that a K3 surface has good reduction if and only if is second cohomology is unramified, and the associated Galois representation over the residue field coincides with the second cohomology of a certain "canonical reduction" of X. This is joint work with B. Chiarellotto and C. Liedtke.
  • Video, Slides
• • 8 July 2020
  • Ed Segal (UCL)
  • Semi-orthogonal decompositions and discriminants
  • The derived category of a toric variety can usually be decomposed into smaller pieces, by passing through different birational models and applying the "windows" theory relating VGIT and derived categories. There are many choices involved and the decompositions are not unique. We prove a Jordan-Holder result, that the multiplicities of the pieces are independent of choices. If the toric variety is Calabi–Yau then there are no decompositions, instead the theory produces symmetries of the derived category. Physics predicts that these all these symmetries form an action of the fundamental group of the "FI parameter space". I'll explain why our Jordan-Holder result is necessary for this prediction to work, and state a conjecture (based on earlier work of Aspinwall-Plesser-Wang) relating our multiplicities to the geometry of the FI parameter space. This is joint work with Alex Kite.
  • Video, Slides
• • 2 July 2020
  • Gregory Smith (Queen’s)
  • Geometry of smooth Hilbert schemes
  • How can we understand the subvarieties of a fixed projective space? Hilbert schemes provide the geometric answer to this question. After surveying some features of these natural parameter spaces, we will classify the smooth Hilbert schemes. Time permitting, we will also describe the geometry of nonsingular Hilbert schemes by interpreting them as suitable generalisations of partial flag varieties. This talk is based on joint work with Roy Skjelnes (KTH).
  • Video, Slides
• • 25 June 2020
  • Klaus Altmann (FU Berlin)
  • Displaying the cohomology of toric line bundles
  • Line bundles L on projective toric varieties can be understood as formal differences (Δ⁺-Δ^-) of convex polyhedra in the character lattice. We show how it is possible to use this language for understanding the cohomology of L by studying the set-theoretic difference (Δ^-\Δ⁺). Moreover, when interpreting these cohomology groups as certain Ext-groups, we demonstrate how the approach via (Δ^-\Δ⁺) leads to a direct description of the associated extensions. The first part is joint work with Jarek Buczinski, Lars Kastner, David Ploog, and Anna-Lena Winz; the second is work in progress with Amelie Flatt.
  • Video
• • 18 June 2020
  • Leonid Monin (Bristol)
  • Inversion of matrices, a C* action on Grassmannians and the space of complete quadrics
  • Let Γ be the closure of the set of pairs (A,A^-1) of symmetric matrices of size n. In other words, Γ is the graph of the inversion map on the space Sym_n of symmetric matrices of size n. What is the cohomology class of Γ in the product of projective spaces? Equivalently, what is the degree of the variety L^-1 obtained as the closure of the set of inverses of matrices from a generic linear subspace L of Sym_n? This question is interesting in its own right but it is also motivated by algebraic statistics. In my talk, I will explain how to invert a matrix using a C* action on Grassmannians, relate the above question to classical enumerative problems about quadrics, and give several possible answers. This is joint work in progress with Laurent Manivel, Mateusz Michalek, Tim Seynnaeve, Martin Vodicka, Andrzej Weber, and Jaroslaw A. Wisniewski.
  • Video
• • 11 June 2020
  • Karin Schaller (FU Berlin)
  • Polyhedral Divisors and Orbit Decompositions of Normal Affine Varieties with Torus Action
  • Normal affine varieties of dimension n with an effective action of a k-dimensional algebraic torus can be described completely in terms of proper polyhedral divisors living on semiprojective varieties of dimension n−k. We use the language of polyhedral divisors to study the collection of T-orbits and T-orbit closures of a normal affine T-variety in terms of its defining pp-divisor. This is based on previous work of Klaus Altmann and Jürgen Hausen complemented by work in progress with Klaus Altmann.
  • Video (YouTube), Video (Vimeo), Slides
• • 4 June 2020
  • Giuliano Gagliardi (Hannover and MPI Bonn)
  • The Manin-Peyre conjecture for smooth spherical Fano varieties of semisimple rank one
  • The Manin-Peyre conjecture is established for a class of smooth spherical Fano varieties of semisimple rank one. This includes all smooth spherical Fano threefolds of type T as well as some higher-dimensional smooth spherical Fano varieties. This is joint work with Valentin Blomer, Jörg Brüdern, and Ulrich Derenthal.
  • Video (YouTube), Video (Vimeo)
• • 28 May 2020
  • Tom Sutherland (Lisbon)
  • Mirror symmetry for Painlevé surfaces
  • This talk will survey aspects of mirror symmetry for ten families of non-compact hyperkähler manifolds on which the dynamics of one of the Painlevé equations is naturally defined. They each have a pair of natural realisations: one as the complement of a singular fibre of a rational elliptic surface and another as the complement of a triangle of lines in a (singular) cubic surface. The two realisations relate closely to a space of stability conditions and a cluster variety of a quiver respectively, providing a perspective on SYZ mirror symmetry for these manifolds.
  • Video (YouTube), Video (Vimeo), Slides
• • 21 May 2020
  • Jesús Martinez Garcia (Essex)
  • The moduli continuity method for log Fano pairs
  • The moduli continuity method, pioneered by Odaka, Spotti and Sun, allows us to explicitly provide algebraic charts of the Gromov–Hausdorff compactification of (possibly singular) Kähler-Einstein metrics. Assuming we can provide a homeomorphism to some 'known' algebraic compactification (customarily, a GIT one) the method allows us to determine which Fano varieties (or more generally log Fano pairs) are K-polystable in a given deformation family. In this talk we provide the first examples of compactification of the moduli of log Fano pairs for the simplest deformation family: that of projective space and a hypersurface, and mention related results for cubic surfaces. This is joint work with Patricio Gallardo and Cristiano Spotti.
  • Video (YouTube), Video (Vimeo), Slides
• • 19 May 2020
  • Timothy Logvinenko (Cardiff)
  • Skein-triangulated representations of generalised braids
  • Ordinary braid group Br_n is a well-known algebraic structure which encodes configurations of n non-touching strands ("braids") up to continuous transformations ("isotopies"). A classical result of Khovanov and Thomas states that there is a natural categorical action of Br_n on the derived category of the cotangent bundle of the variety of complete flags in Cⁿ. In this talk, I will introduce a new structure: the category GBr_n of generalised braids. These are the braids whose strands are allowed to touch in a certain way. They have multiple endpoint configurations and can be non-invertible, thus forming a category rather than a group. In the context of triangulated categories, it is natural to impose certain relations which result in the notion of a skein-triangulated representation of GBr_n. A decade-old conjecture states that there a skein-triangulated action of GBr_n on the cotangent bundles of the varieties of full and partial flags in Cⁿ. We prove this conjecture for n = 3. We also show that any categorical action of Br_n can be lifted to a skein-triangulated action of GBr_n, which behaves like a categorical nil Hecke algebra. This is a joint work with Rina Anno and Lorenzo De Biase.
  • Video (YouTube), Video (Vimeo)
• • 14 May 2020
  • Alan Thompson (Loughborough)
  • Threefolds fibred by sextic double planes
  • I will discuss the theory of threefolds fibred by K3 surfaces mirror to the sextic double plane. This theory is unexpectedly rich, in part due to the presence of a polarisation-preserving involution on such K3 surfaces. I will present an explicit birational classification result for such threefolds, along with computations of several of their basic invariants. Along the way we will uncover several (perhaps) surprising links between this theory and Kodaira's theory of elliptic surfaces. This is joint work with Remkes Kooistra.
  • Video (YouTube), Video (Vimeo)
• • 13 May 2020
  • Tom Ducat (Imperial)
  • A Laurent phenomenon for OGr(5,10) and explicit mirror symmetry for the Fano 3-fold V₁₂
  • The 5-periodic birational map (x, y) -> (y, (1+y)/x) can be interpreted as a mutation between five open torus charts in a del Pezzo surface of degree 5, coming from a cluster algebra structure on the Grassmannian Gr(2,5). This can used to construct a rational elliptic fibration which is the Landau-Ginzburg mirror to dP₅. I will briefly recap this, and then explain the following 3-dimensional generalisation: the 8-periodic birational map (x, y, z) -> (y, z, (1+y+z)/x) can be used to exhibit a Laurent phenomenon for the orthogonal Grassmannian OGr(5,10) and construct a completely explicit K3 fibration which is mirror to the Fano 3-fold V₁₂, as well as some other Fano 3-folds.
  • Video (YouTube), Video (Vimeo), Slides
• • 6 May 2020
  • Livia Campo (Nottingham)
  • On a high pliability quintic hypersurface
  • In this talk we exhibit an example of a quintic hypersurface with a certain compound singularity that has pliability at least 2. We also show that, while a non-trivial sequence of birational transformations can be constructed between the two elements of the pliability set, the Sarkisov link connecting them is not evident. This is done by studying birational links for codimension 4 index 1 Fano 3-folds having Picard rank 2.
• • 30 April 2020
  • Florian Kohl (Aalto)
  • Unconditional reflexive polytopes
  • A convex body is unconditional if it is symmetric with respect to reflections in all coordinate hyperplanes. In this talk, we investigate unconditional lattice polytopes with respect to geometric, combinatorial, and algebraic properties. In particular, we characterise unconditional reflexive polytopes in terms of perfect graphs. As a prime example, we study a type-B analogue of the Birkhoff polytope. This talk is based on joint work with McCabe Olsen and Raman Sanyal.
  • Slides
• • 16 April 2020
  • Alessio Corti (Imperial)
  • Volume preserving birational selfmaps of P³
  • I describe some results on the structure of the group Bir(P³, D) where D is a quartic surface with mild singularities. Work with Carolina Araujo and Alex Massarenti.
• • 8 April 2020
  • Thomas Prince (Oxford)
  • Perturbing torus fibrations on threefold singularities
  • Fix a Q-Gorenstein threefold toric singularity X determined by a rational polyhedral cone C in N_R, together with a collection of piecewise linear maps (or combinatorial mutations) T₁,...,T_k such that the image of C* under the composition of these linear maps is a half space in R³. We describe how to perturb the torus fibration X to C*, whose fibres are orbits of the (toric) T³ action on X, to a torus fibration on a space X' which is a manifold away from a finite collection of singular points. Around each of these singular points X' has the structure of a terminal cyclic quotient singularity. We outline how to globalise this to construct torus fibrations over 3-dimensional balls which correspond to (partially) smoothing a three-dimensional toric Fano variety to a Fano variety with cyclic quotient terminal singularities. The combinatorial input to this process is closely related to the notion of zero-mutable Laurent polynomial which has been recently studied by Corti, Kasprzyk, and Pitton.
• • 2 April 2020
  • Alice Cuzzucoli (Warwick)
  • A glimpse at the classification of orbifold del Pezzo surfaces
  • In this talk, we will discuss the main ingredients involved in the classification of del Pezzo surfaces with orbifold points, i.e. complex projective varieties of dimension two admitting log terminal singularities. In the smooth case, we have a well-known birational classification dating back to the 19th century. In the singular case, we are still missing a classification just as complete. Nevertheless, in the case of cyclic quotient singularities, we have some interesting constructions. We will introduce the most crucial aspects of such constructions, which are divided into three main steps: firstly, by analysing the graded rings of such surfaces, we can find a bound on the number of singularities and the relative invariants; secondly, with the help of Mori Theory, we can give a first representation of our birational models; then, by having a brief look at the toric case, we will describe how toric degenerations come into play in this classification. Ultimately, we can recreate analogous constructions to the cascade of blow ups for the smooth case with the representatives of specific deformation classes of our orbifolds.
  • Slides

Past Reading Groups

• • 24 August - 30 November 2020, Mondays
  • Tropical Combinatorics and Geometry
  • A weekly reading group working through M. Joswig's book draft "Essentials of Tropical Combinatorics". In addition, we will cover sections from "Brief Introduction to Tropical Geometry" (E. Brugalle, I. Itenberg, G. Mikhalkin, K. Shaw) and "Tropical Data Science" (R. Yoshida). Material presented by Marie-Charlotte Brandenburg, Giulia Codenotti, Maria Dostert, Danai Deligeorgaki, Girtrude Hamm, Aryaman Jal, Katharina Jochemko, Florian Kohl, Fatemeh Mohammadi, Leonid Monin, Petter Restadh, Felix Rydell, and Leonardo Saud.
  • Note: This reading group is recognised as an official seminar worth 7.5 credits for students at KTH. For further information, please visit the reading group's KTH webpage.
• • 1 September - 24 November 2020, Tuesdays
  • Birational Geometry
  • A weekly reading group working through K. Matsuki's book "Introduction to the Mori Program". Material presented by Livia Campo, Lucas Das Dores, Giuliano Gagliardi, Tiago Guerreiro, Thomas Hall, Johannes Hofscheier, ​Jesus Martinez-Garcia, ​Leonid Monin, and Theodoros Papazachariou.
• • 31 March - 7 July 2020, Tuesdays
  • Geometry of Numbers
  • A weekly reading group working through Siegel's "Lectures on the Geometry of Numbers". Material presented by Livia Campo, Daniel Cavey, Giulia Codenotti, Oliver Daisey, Thomas Hall, Johannes Hofscheier, Katharina Jochemko, and Leonid Monin.