Online Algebraic Geometry Seminar
This seminar is held online via Microsoft Teams on the Nottingham University "Algebraic Geometry" team. To be added to this team, please email one of Alex Kasprzyk, Livia Campo, or Johannes Hofscheier. For help joining a talk, please follow these instructions. Everybody is welcome.
Note: All times are UK times.
Subscribe to the online calendar to be kept uptodate about upcoming talks (also available in iCal format). Recordings of talks are available.
Upcoming Talks

 15 April 2021, 12pm
 Johannes Nordstrom (Bath)
 Extratwisted connected sum G_{2}manifolds
 The twisted connected sum construction of Kovalev produces many examples of closed Riemannian 7manifolds with holonomy group G_{2} (a special class of Ricciflat manifolds), starting from complex algebraic geometry data like Fano 3folds. 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 7manifolds with disconnected moduli space of holonomy G_{2} metrics, or pairs of G_{2}manifolds that homeomorphic but not diffeomorphic. This is joint work with Diarmuid Crowley and Sebastian Goette.
 Link to event

 22 April 2021, 12pm
 Ben Wormleighton (Washington)
 TBA

 29 April 2021, 1011am
 Michael Wemyss (Glasgow)
 TBA

 5 May 2021, 34pm
 Travis Mandel (Oklahoma)
 TBA
Past Talks

 8 April 2021, 1011am
 Tim Gräfnitz (Hamburg)
 Tropical correspondence for smooth del Pezzo log CalabiYau pairs
 In this talk I will present the main results of my thesis, a tropical correspondence theorem for log CalabiYau 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 (P^{2},E), where E is an elliptic curve. I will explain how the genus zero logarithmic GromovWitten 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 GrossSiebert program. The novelty in this correspondence is that D is smooth but nontoric, leading to log singularities in the toric degeneration that have to be resolved.
 Video, Slides

 1 April 2021, 12pm
 JiaRui Fei (Shanghai Jiao Tong)
 Tropical Fpolynomials and Cluster Algebras
 The representationtheoretic interpretations of gvectors and Fpolynomials are two fundamental ingredients in the (additive) categorification of cluster algebras. We knew that the gvectors are related to the presentation spaces. We introduce the tropical Fpolynomial f_{M} of a quiver representation M, and explain its interplay with the general presentation for any finitedimensional 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 representationtheoretic interpretation of FockGoncharov's cluster duality pairing. We also study many combinatorial aspects of N(M), such as faces, the dual fan and 1skeleton. 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 clusterfinite algebras and the preprojective algebras of Dynkin type.
 Video, Slides

 25 March 2021, 1011am
 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, Slides

 18 March 2021, 1011am
 Taro Sano (Kobe)
 Construction of nonKähler CalabiYau manifolds by log deformations
 CalabiYau 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 nonKähler CalabiYau 3folds. 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 CalabiYau manifolds of Schoen type.
 Video, Slides

 11 March 2021, 12pm
 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, Slides

 4 March 2021, 1011am
 Fatemeh Rezaee (Loughborough)
 Wallcrossing does not induce MMP
 I will describe a new wallcrossing phenomenon of sheaves on the projective 3space 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

 2326 February 2021
 Fano Varieties and Birational Geometry
 An online workshop exploring recent developments in the geometry of Fano varieties.
 Workshop webpage

 18 February 2021, 1011am
 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 1forms and of the relative top forms. We show the existence of higher irrational pencils under natural hypothesis on local subsystems.
 Video, Slides

 11 February 2021, 11am12pm
 Enrica Mazzon (Bonn)
 Nonarchimedean approach to mirror symmetry and to degenerations of varieties
 Mirror symmetry is a fastmoving research area at the boundary between mathematics and theoretical physics. Originated from observations in string theory, it suggests that complex CalabiYau manifolds should come in mirror pairs, in the sense that geometrical information of a CalabiYau 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 nonarchimedean geometry and the theory of Berkovich spaces. In the second part, I will describe a combinatorial object, the socalled 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 PilleSchneider.
 Video, Slides

 4 February 2021, 1011am
 Pieter Belmans (Bonn)
 Hochschild cohomology of partial flag varieties and Fano 3folds
 The HochschildKostantRosenberg 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 nontrivial 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 3folds (joint work with Enrico Fatighenti and Fabio Tanturri).
 Video, Slides

 28 January 2021, 1011am
 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

 21 January 2021, 1011am
 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 CalabiYau surface obtained by twisting Y by the sum of the line bundles dual to the components of the boundary; 3) the all genus open GromovWitten theory of a toric CalabiYau threefold associated to the Looijenga pair; 4) the DonaldsonThomas 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 closedform 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, Slides

 14 January 2021, 34pm
 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, Slides

 10 December 2020, 4.305.30pm
 Chris Eur (Stanford)
 Tautological bundles of matroids
 Recent advances in matroid theory via tropical geometry broadly fall into two themes: One concerns the Ktheory 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 logconcavity statements, and answer few conjectures in the literature. This is an ongoing project with Andrew Berget, Hunter Spink, and Dennis Tseng.
 Video, Slides

 3 December 2020, 1.302.30pm
 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 NewtonOkounkov bodies and 2) a possible cluster version of a classic toric mirror symmetry construction due to Batyrev. Based on joint work with ManWai Cheung and Alfredo Nájera Chávez.
 Video, Slides

 26 November 2020, 1.302.30pm
 Okke van Garderen (Glasgow)
 Refined DonaldsonThomas theory of threefold flops
 DT invariants are virtual counts of semistable objects in the derived category of a CalabiYau 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 wallcrossing formulas, and use this to reduce the DT theory of a flop to a comprehensible set of curvecounting invariants, which can be computed for several examples. These computations produce new evidence for a conjecture of PandharipandeThomas, and show that refined DT invariants are not enough to completely classify flops.
 Video, Slides

 20 November 2020, 1011am
 Ana PeónNieto (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, 1011am
 Naoki Fujita (Tokyo)
 NewtonOkounkov 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, NewtonOkounkov bodies, and cluster algebras. More precisely, we construct NewtonOkounkov bodies using cluster structures, and realize representationtheoretic and clustertheoretic toric degenerations from this framework. As an application, we connect two kinds of representationtheoretic polytopes (string polytopes and NakashimaZelevinsky 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 representationtheoretic polytopes by combinatorial mutations. This talk is based on joint works with Hironori Oya and Akihiro Higashitani.
 Video, Slides

 12 November 2020, 1.302.30pm
 Arkadij Bojko (Oxford)
 Orientations for DT invariants on quasiprojective CalabiYau 4folds
 DonaldsonThomas 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 CalabiYau 4folds. 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 welldefined, one needs to prove orientability. The result of CaoGrossJoyce does this for projective CY 4folds. However, computations are more feasible in the noncompact 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 CalabiYau 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 Ktheory and satisfy compatibility under direct sums. If time allows, I will discuss the connection between the sings obtained from comparing orientations and universal wallcrossing formulae of Joyce using vertex algebras.
 Video, Slides

 11 November 2020, 1011am
 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 3folds. In this talk, we give a survey of some recent progress along the same lines, including a biregular rework of the nonprime MoriMukai 3folds classification and some examples of higherdimensional Fano varieties with special Hodgetheoretical properties.
 Video, Slides

 5 November 2020, 1.302.30pm
 Federico Barbacovi (UCL)
 Understanding the flopflop 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 pullpush via the fibre product. When this approach work, we obtain an interesting autoequivalence of either side of the flop, known as the "flopflop 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 "flopflop 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, Slides

 29 October 2020, 23pm
 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 DonaldsonFukayaSeidel 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 4torus is SYZ mirror to a 4torus. So if we view the complex genus 2 curve as a hypersurface of a 4torus V, a mirror can be constructed as a symplectic fibration with fiber given by the dual 4torus V^. Hence on categories, line bundles on V are restricted to the genus 2 curve while fiber Lagrangians of V^ are parallel transported over Ushapes in the base of the mirror. Next we describe ongoing work with H. Azam, H. Lee, and CC. M. Liu on extending the result to a global statement, namely allowing the complex and symplectic structures to vary in their real sixdimensional families. The mirror statement for this more general result relies on work of A. Kanazawa and SC. Lau.
 Video, Slides

 22 October 2020, 34pm
 Tyler Kelly (Birmingham)
 What is an exoflop?
 Aspinwall stated in 2014 that an exoflop "occurs in the gauged linear sigmamodel 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

 15 October 2020, 12pm
 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 quasihomogeneous and quasismooth (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 KreuzerSkarke classification of invertible polynomials as Thom—Sebastiani sums of Fermat, chain, and loop polynomials. I’ll present a friendly, exampleoriented illustration of our approach, review related literature, and discuss applications to mirror symmetry.
 Video, Slides

 7 October 2020, 23pm
 Hiroshi Iritani (Kyoto)
 Quantum cohomology of blowups: a conjecture
 In this talk, I discuss a conjecture that a semiorthogonal decomposition of topological Kgroups (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 RiemannHilbert problem.
 Slides, Handout

 1 October 2020, 34pm
 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, Slides

 24 September 2020, 34pm
 Navid Nabijou (Cambridge)
 Degenerating tangent curves
 It is wellknown 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 GromovWitten 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 GromovWitten 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 GromovWitten theory, but they do not appear to admit a classical proof. No prior knowledge of GromovWitten theory will be assumed.
 Video, Slides

 17 September 2020, 1011am
 Ronan Terpereau (Bourgogne)
 Actions of connected algebraic groups on rational 3dimensional 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(P^{3}) obtained by Hiroshi Umemura in the 1980's when the base field is the field of complex numbers.
 Video, Slides

 10 September 2020, 45pm
 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 mspace that assembles the Gröbner degenerations of V associated with all faces of C. This is a multiparameter generalization of the classical oneparameter Gröbner degeneration associated to a weight. We show that our family can be constructed from KavehManon'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 StanleyReisner 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 EscobarHarada's mutation of NewtonOkounkov bodies can be recovered as tropicalized cluster mutation. This is joint work with Fatemeh Mohammadi and Alfredo Nájera Chávez.
 Video, Slides

 4 September 2020, 34pm
 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 nondegenerate log 2form 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 CalabiYau 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, 34pm
 Renato Vianna (Rio de Janeiro)
 Sharp ellipsoid embeddings and almosttoric mutations
 We will show how to construct volume filling ellipsoid embeddings in some 4dimensional toric domain using mutation of almost toric compactification of those. In particular we recover the results of
McDuffSchlenk for the ball, FenkelMüller for product of symplectic disks and CristofaroGardiner 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, 1.302.30pm
 Andrea Petracci (FU Berlin)
 Kmoduli stacks and Kmoduli spaces are singular
 Only recently a separated moduli space for (some) Fano varieties has been constructed by several algebraic geometers: this is the Kmoduli stack which parametrises Ksemistable 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 nonsmooth points in the Kmoduli stack and the Kmoduli space of Fano 3folds. This is joint work with AnneSophie Kaloghiros.
 Video, Slides

 20 August 2020, 23pm
 ManWai "Mandy" Cheung (Harvard)
 Polytopes, wall crossings, and cluster varieties
 Cluster varieties are log CalabiYau 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 nonintegral 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, NajeraChavez, and Vienna.
 Video, Slides

 13 August 2020, 45pm
 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 ngon. For type D root systems, one obtains a polytope parametrizing centrally symmetric triangulations of a 2ngon. In previous work, Jan Christophersen and I showed that the StanleyReisner 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 workinprogress that generalizes this unobstructedness result to the type D associahedron.
 Video, Slides

 6 August 2020, 1011am
 YangHui He (City and Oxford)
 Universes as big data: superstrings, CalabiYau manifolds and machinelearning
 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 CalabiYau landscape, accumulated by the collaboration of physicists, mathematicians and computer scientists over the last 4 decades, we show how the latest techniques in machinelearning can help explore problems of physical and mathematical interest.
 Video, Slides

 30 July 2020, 34pm
 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 higherorder 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, 45pm
 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 wellunderstood. 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 GelfandCetlin 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, 1.302.30pm
 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 Kstability. 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, 1011am
 Chris Lazda (Warwick)
 A NeronOggShafarevich criterion for K3 surfaces
 The naive analogue of the NéronOggShafarevich 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 semistable 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, 1.302.30pm
 Ed Segal (UCL)
 Semiorthogonal 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 JordanHolder result, that the multiplicities of the pieces are independent of choices. If the toric variety is CalabiYau 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 JordanHolder result is necessary for this prediction to work, and state a conjecture (based on earlier work of AspinwallPlesserWang) relating our multiplicities to the geometry of the FI parameter space. This is joint work with Alex Kite.
 Video, Slides

 2 July 2020, 1.302.30pm
 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, 1011am
 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 settheoretic difference (Δ^{}\Δ^{+}). Moreover, when interpreting these cohomology groups as certain Extgroups, 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 AnnaLena Winz; the second is work in progress with Amelie Flatt.
 Video

 18 June 2020, 1.302.30pm
 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, 1011am
 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 kdimensional 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 Torbits and Torbit closures of a normal affine Tvariety in terms of its defining ppdivisor. This is based on previous work of Klaus Altmann and Jürgen Hausen complemented by work in progress with Klaus Altmann.
 Video, Slides

 4 June 2020, 1.302.30pm
 Giuliano Gagliardi (Hannover and MPI Bonn)
 The ManinPeyre conjecture for smooth spherical Fano varieties of semisimple rank one
 The ManinPeyre 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 higherdimensional smooth spherical Fano varieties. This is joint work with Valentin Blomer, Jörg Brüdern, and Ulrich Derenthal.
 Video

 28 May 2020, 1011am
 Tom Sutherland (Lisbon)
 Mirror symmetry for Painlevé surfaces
 This talk will survey aspects of mirror symmetry for ten families of noncompact 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, Slides

 21 May 2020, 1.302.30pm
 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 GromovHausdorff compactification of (possibly singular) KählerEinstein 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 Kpolystable 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, Slides

 19 May 2020, 23pm
 Timothy Logvinenko (Cardiff)
 Skeintriangulated representations of generalised braids
 Ordinary braid group Br_{n} is a wellknown algebraic structure which encodes configurations of n nontouching 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 C^{n}. 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 noninvertible, 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 skeintriangulated representation of GBr_{n}. A decadeold conjecture states that there a skeintriangulated action of GBr_{n} on the cotangent bundles of the varieties of full and partial flags in C^{n}. We prove this conjecture for n = 3. We also show that any categorical action of Br_{n} can be lifted to a skeintriangulated 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

 14 May 2020, 1011am
 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 polarisationpreserving 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

 13 May 2020, 12pm
 Tom Ducat (Imperial)
 A Laurent phenomenon for OGr(5,10) and explicit mirror symmetry for the Fano 3fold V_{12}
 The 5periodic 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 LandauGinzburg mirror to dP_{5}. I will briefly recap this, and then explain the following 3dimensional generalisation: the 8periodic 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 3fold V_{12}, as well as some other Fano 3folds.
 Video, Slides

 6 May 2020, 1011am
 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 nontrivial 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 3folds having Picard rank 2.

 30 April 2020, 9.3010.30am
 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 typeB analogue of the Birkhoff polytope. This talk is based on joint work with McCabe Olsen and Raman Sanyal.
 Slides

 16 April 2020, 1011am
 Alessio Corti (Imperial)
 Volume preserving birational selfmaps of P^{3}
 I describe some results on the structure of the group Bir(P^{3}, D) where D is a quartic surface with mild singularities. Work with Carolina Araujo and Alex Massarenti.

 8 April 2020, 1011am
 Thomas Prince (Oxford)
 Perturbing torus fibrations on threefold singularities
 Fix a QGorenstein 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) T_{1},...,T_{k} such that the image of C* under the composition of these linear maps is a half space in R^{3}. We describe how to perturb the torus fibration X to C*, whose fibres are orbits of the (toric) T^{3} 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 3dimensional balls which correspond to (partially) smoothing a threedimensional 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 zeromutable Laurent polynomial which has been recently studied by Corti, Kasprzyk, and Pitton.

 2 April 2020, 1011am
 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 wellknown 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, 12.302pm
 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 MarieCharlotte 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, 34pm
 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 MartinezGarcia, Leonid Monin, and Theodoros Papazachariou.

 31 March  7 July 2020, Tuesdays, 9.3010.30am
 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.