UC Riverside Algebraic Geometry Seminar

UC Riverside Algebraic Geometry Seminar


The UC Riverside Algebraic Geometry Seminar meets on Tuesdays from 11:00am to 12:00pm. We usually meet in Skye 268, but the meetings are online via Zoom during Winter 2022. For more information you may contact Ziv Ran (ziv.ran@ucr.edu), Jose Gonzalez (jose.gonzalez@ucr.edu) or Patricio Gallardo (patricio.gallardocandela@ucr.edu). Please find our schedule below. For information about reimbursements for our visitors click here.



Spring 2022

Date Speaker Title Abstract
Tuesday, April 12, 2022
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Patricio Gallardo
UC Riverside
Variation of stability for moduli spaces of unordered points in the plane. A central insight of algebraic geometry is that a moduli space has many geometrically meaningful compactifications and that much of its birational geometry can be understood via the degenerations of the objects these compactifications are parametrizing. Describing such interplay is one of the leading questions in moduli theory nowadays. Within this context, we report ongoing work with Benjamin Schmidt on compactifications of the moduli of unlabelled points in the plane. Our spaces are obtained via GIT quotients of both the Hilbert scheme and its birational models constructed with Bridgeland stability. In particular, we describe the role of non-reduced points and the existence of a compactification that is the opposite of the familiar Chow quotient.
Tuesday, April 26, 2022
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Cesar Lozano
UNAM Mexico
Birational geometry of the moduli space of sheaves on the plane via minimal free resolutions. The minimal free resolution of a coherent sheaf on the plane is a classical tool in algebraic geometry which dates back to David Hilbert early in the 20th century. The purpose of this talk is to show the audience that minimal free resolutions can be used to study the birational geometry of the moduli space of coherent sheaves on the plane; which has so far been studied through Bridgeland stability. The talk will begin by describing the effective and movable cones of the Hilbert scheme of n points on the plane P^2[n], using minimal free resolutions. We will do so in a concrete and human-oriented manner. If time permits, we will discuss how to recover the relevant Bridgeland destabilizing objects from minimal free resolutions in order to recover the stable base locus decomposition of P^2[n], for small values of n.
Tuesday, May 3, 2022
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Mihai Fulger
University of Connecticut
Positivity vs. slope semistability for bundles with vanishing discriminant. Positivity and semistability are important tools in projective algebraic geometry. They share some similar flavors. On curves, Hartshorne proved that a vector bundle is semistable iff its normalization is nef. The correct generalization to higher dimension of this result is that a bundle with vanishing discriminant is slope-semistable iff its normalization is nef. This was known, but previous proofs either use nonabelian Hodge theory in characteristic 0, or make significant use of the Frobenius morphism in positive characteristic. We give a characteristic free proof. We also address a recent question of S. Misra. This is in joint work with A. Langer.
Tuesday, May 10, 2022
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Yilong Zhang
Ohio State University
Hilbert scheme of a pair of skew lines on cubic threefold. We study the irreducible component of the Hilbert scheme of a cubic threefold determined by a pair of skew lines. We show that the component is smooth and isomorphic to the blow-up on the diagonal of the 2nd symmetric product of the Fano surface of lines of cubic threefold. This work is based on the study Chen-Coskun-Nollet on the Hilbert scheme of a pair of skew lines on projective spaces in 2011 and the Abel-Jacobi map on cubic threefold by Clemens and Griffiths. Moreover, we'll explore the relation to the moduli of sheaves on cubic threefold considered by Bayer et al. and Bridgeland moduli space considered by Altavilla, Petkovic, and Rota.
Tuesday, May 17, 2022
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Mark Shoemaker
Colorado State University
Enumerative Geometry and Mirror Symmetry. The goal of enumerative geometry is to study a geometric space by counting certain subspaces within it. The first result in enumerative geometry is Euclid’s observation that, given 2 distinct points in the plane, there is a single line through these points. A harder question is, given 2 points and 3 random lines in the plane how many conics (degree 2 curves) pass through both points and are tangent to each of the lines. These types of questions have interested geometers since the 1800's and earlier, but they are famously difficult. However, a breakthrough occurred in the 1990’s when a surprising connection was made with physics. It was discovered that techniques and intuitions from string theory could be used to answer longstanding questions in enumerative geometry. The phenomenon behind this remarkable connection came to be known as mirror symmetry. In this talk I will give an introduction to mirror symmetry and its connection to enumerative geometry. At the end of the talk I will mention some current directions of inquiry and open questions.
Tuesday, May 24, 2022
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Samir Canning
UC San Diego
The Chow rings of moduli spaces of elliptic surfaces. For each nonnegative integer N, Miranda constructed a coarse moduli space of elliptic surfaces with section over the projective line with fundamental invariant N. I will explain how to compute the Chow rings with rational coefficients of these moduli spaces when N is at least 2. The Chow rings exhibit many properties analogous to those expected for the tautological ring of the moduli space of curves: they satisfy analogues of Faber's conjectures, and they exhibit a stability property as N goes to infinity. When N=2, these elliptic surfaces are K3 surfaces polarized by a hyperbolic lattice. I will explain how the computation of the Chow ring confirms a special case of a conjecture of Oprea and Pandharipande on the structure of the tautological rings of moduli spaces of lattice polarized K3 surfaces. This is joint work with Bochao Kong.
Tuesday, May 31, 2022
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Emily Clader
San Francisco State University
Permutohedral Complexes and Curves With Cyclic Action. There is a beautiful combinatorial and geometric story connecting a polytope known as the permutohedron, the algebra of the symmetric group, and the geometry of a particular moduli space of curves first studied by Losev and Manin. I will describe these three worlds and their connection to one another, and then I will discuss joint work with C. Damiolini, D. Huang, S. Li, and R. Ramadas that generalizes the story by introducing a new family of polytopal complexes and relating it to a new family of moduli spaces.



Winter 2022

Date Speaker Title Abstract
Tuesday, January 11, 2022
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Patricio Gallardo
UC Riverside
An introduction to Chow quotients. In this talk aimed to graduate students, we will describe the Chow quotient construction, a technique that considers orbits of a group action and its degenerations, for compactifying a moduli space. We will discuss key structural theorems, recent applications, and a couple of open problems.
Tuesday, January 18, 2022
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Javier Gonzalez Anaya
UC Riverside
Introduction to divisors. This talk is an introductory survey on the theory of divisors on varieties. We introduce both Weil and Cartier divisors, their main features, and related constructions. Some relevant examples will be discussed along the way.
Tuesday, January 25, 2022
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Eloísa Grifo
University of Nebraska
Bounding the degree of higher order vanishing. Given a finite set of points in projective space, what is the smallest degree of a hypersurface passing through each point with a given multiplicity? In this talk, we will discuss a conjectured lower bound by Chudnovsky, which was recently shown to hold for sufficiently large sets of general points. This is joint work with Sankhaneel Bisui, Tái Hà, and Thái Thành Nguyễn.
Tuesday, February 1, 2022
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Olivia Dumitrescu
University of North Carolina
On the theory of curves in Pn. In this talk we will present the Weyl group action on curves in the projective space blown up in s points in general position. In particular, we will construct examples of rigid curves that we call (-1) curves in Pn; we further prove that (-1) curves on blown up P3 in points can be equivalently defined arithmetically by a linear and a quadratic invariant. We investigate moving curves in Pn that we call (0) and (1) curves and we explore their applications. This talk is based on the work with Rick Miranda.
Tuesday, February 8, 2022
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Jake Levinson
Simon Fraser University
Degenerations for products of psi and omega classes on M0,n. I will describe recent work, joint with Maria Gillespie and Sean Griffin, on products of psi and omega divisor classes on the moduli space of stable curves of genus zero. We construct a flat family of subschemes of M0,n. whose general fiber is a complete intersection representing the product, and whose special fiber is a generically reduced union of boundary strata. Our family uses an explicit set of parametrized hyperplane equations, and the resulting strata are enumerated by a combinatorial rule we called "slide labeling" on trees. As such, we express these products as positive, multiplicity-free sums of boundary classes in the Chow ring. As a corollary, we also express kappa classes as positive sums of boundary strata.
Tuesday, February 15, 2022
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Lara Bossinger
Mathematics Institute of the National Autonomous University of Mexico (UNAM)
Gröbner theory of Grassmannian cluster algebras. In this talk I will report on joint work with Fatemeh Mohammadi and Alfredo Nájera Chávez. For a (multi-)homogeneous ideal inside a polynomial ring we generalize classical Gröbner degenerations to define a multi-parameter family combining all Gröbner degenerations associated with a maximal cone in the Gröbner fan and all its faces. When this construction is applied to the homogeneous coordinate ring of the Grassmannians Gr(2,n) and Gr(3,6) (presented compatible with their cluster structure) the algebra defining the family for a certain maximal Gröbner cone coincides with the universal coefficient cluster algebra. I will review the basic notions needed from Gröbner theory and cluster algebras, explain our general construction and our (computational) results for the Grassmannians.
Tuesday, February 22, 2022
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Simon Telen
Max Planck Institute
Likelihood equations and scattering amplitudes. We identify the scattering equations from particle physics as the likelihood equations for a particular statistical model. The scattering potential plays the role of the log-likelihood function. We employ recent methods from numerical algebraic geometry for solving rational function equations to compute and certify all critical points, and show that the same methods can be used to study the ML degree of low rank tensor models. We revisit physical theories proposed by Arkani-Hamed, Cachazo and their collaborators, introducing positive statistical models and their string amplitudes. This is joint work with Bernd Sturmfels.
Tuesday, March 1, 2022
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
No meeting this day.
No meeting this day. No meeting this day.



Fall 2021

Date Speaker Title Abstract
Tuesday, October 12, 2021
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Kiumars Kaveh
University of Pittsburgh
Toric schemes over a DVR, vector bundles and affine buildings. This is a report on a work in progress with Chris Manon and Boris Tsvelikhovsky. Motivated by problems in arithmetic geometry, it is natural to consider algebraic varieties defined over a discrete valuation ring (DVR). After recalling some basic concepts about toric varieties, we review the classification of toric schemes defined over a DVR (going back to Mumford in the 70s). We will then discuss our new results on classification of equivariant vector bundles on toric schemes in terms of “piecewise affine maps” to the Bruhat-Tits building of GL(n).
Tuesday, October 26, 2021
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Giancarlo Urzua
Pontifical Catholic University of Chile
About a mysterious continued fraction. On the one hand and in relation to negative curves in the blow-up of a weighted projective plane at a general point, one can see in the paper https://arxiv.org/pdf/2002.07123.pdf page 17 an infinite negative continued fraction. It codifies some families of surfaces with a negative curve. On the other hand and in relation to MMP for degenerations of surfaces, one can see in the paper https://arxiv.org/pdf/1310.1580.pdf page 36 the same infinite continued fraction. It also codifies families of surfaces with negative curves but in the context of MMP. We (I) do not know if there are any connections. The purpose of this talk is to describe the MMP side of the story as much as we can in 50 minutes.
Tuesday, November 2, 2021
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Han-Bom Moon
Fordham University
Point configurations, phylogenetic trees, and dissimilarity maps. In 2004 Pachter and Speyer introduced the dissimilarity maps for phylogenetic trees and asked two important questions about their relationship with tropical Grassmannian. Multiple authors answered affirmatively the first of these questions, showing that dissimilarity vectors lie on the tropical Grassmannian, but the second question, whether the set of dissimilarity vectors forms a tropical subvariety, remained open. In this talk, we present a weighted variant of the dissimilarity map and show that weighted dissimilarity vectors form a tropical subvariety of the tropical Grassmannian in exactly the way that Pachter-Speyer envisioned. This tropical variety has a geometric interpretation in terms of point configurations on rational normal curves. This is joint work with Alessio Caminata, Noah Giansiracusa, and Luca Schaffler.
Tuesday, November 9, 2021
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Haohua Deng
Washington University in St. Louis
Extension of period maps by polyhedral fans. After Griffiths’s fundamental works on the theory of periods, completions of period maps coming from algebraic geometry have been studied for decades. For period domains of classical types on which the Griffiths Transversality condition is trivial, there are many well-known results, for example, Baily-Borel, Mumford, Looijenga, etc. However, completing period maps of non-classical types remains as a widely open field. In this talk, we will briefly review the literature, and take a close look on Kato-Usui’s construction which gives a generalization of Mumford’s toroidal compactification to non-classical cases. We will show some key properties and obstructions of Kato-Usui’s theory, and introduce an application to a non-classical motivic variation with 2 parameters worked out by the speaker.
Tuesday, November 16, 2021
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Angelica Cueto
Ohio State University
Splice type surface singularities and their local tropicalizations. Splice type surface singularities were introduced by Neumann and Wahl as a generalization of the class of Pham-Brieskorn-Hamm complete intersections of dimension two. Their construction depends on a weighted graph with no loops called a splice diagram. In this talk, I will report on joint work with Patrick Popescu-Pampu and Dmitry Stepanov (arXiv: 2108.05912) that sheds new light on these singularities via tropical methods, reproving some of Neumann and Wahl's earlier results on these singularities, and showings that splice type surface singularities are Newton non-degenerate in the sense of Khovanskii.
Tuesday, November 23, 2021
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Theodoros Papazachariou
University of Essex
GIT and K-stability for Fano varieties. In algebraic geometry, one studies varieties which occur as solutions to polynomial equations. In particular, we deal with projective varieties which are the solution spaces of homogeneous polynomials. An important category of geometric objects in algebraic geometry is smooth Fano varieties, which are varieties with positive curvature. As such they can be thought of as higher dimensional analogues of the sphere. These have been classified in 1, 10 and 105 families for curves, surfaces and threefolds respectively, while in higher dimensions the number of Fano families is yet unknown, although we know that their number is bounded. An important current problem is compactifying these families into moduli spaces, i.e., spaces which parametrise objects with some common properties. The aim for the above is so that we can study these families into more details. In this talk I will discuss how one can obtain such compactifications using Geometric Invariant Theory (GIT), which studies (algebraic) group actions on varieties. I will also discuss how one can get similar compactifications using the theory of K-stability, and the links this has to GIT.
Tuesday, November 30, 2021
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Aaron Goodwin
UC Riverside
The Toric Geometry of Quiver Representations. Many difficult linear algebra problems can be posed as classifying equivalent representations of a directed graph, called a quiver. Using geometric invariant theory we study the geometry of these problems, and how they depend upon the choice of a character of a certain group. We will also discuss certain conditions which imply that the moduli spaces of representations of our quiver are all toric varieties, so their classification can be understood combinatorially.



Spring 2021

Date Speaker Title Abstract
Tuesday, April 6, 2021
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Javier Gonzalez-Anaya
UC Riverside
A review of the theory of varieties. This is an introductory talk for the graduate students who will attend the seminar this quarter. We will review the theory of varieties: their construction, the Zariski topology and maps between them. We'll provide examples along the way to illustrate these concepts.    Slides
Tuesday, April 13, 2021
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Rohini Ramadas
Brown University
Dynamics on the moduli space M0,n. A rational function f(z) in one variable determines a self-map of P1. A rational function is called post-critically finite (PCF) if every critical (ramification) point is either pre-periodic or periodic. PCF rational functions have been studied for their special dynamics, and their special distribution within the moduli space of all rational maps. By works of W. Thurston and S. Koch, every PCF map (with a well-understood class of exceptions) arises as an isolated fixed point of an algebraic dynamical system on the moduli space M_{0,n} of point-configurations on P^1; these dynamical systems are called Hurwitz correspondences. I will introduce Hurwitz correspondences and their connection to PCF rational maps, and discuss how the dynamical complexity of Hurwitz correspondence can be studied via combinatorial compactifications of M_{0,n}.
Tuesday, April 20, 2021
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Julie Rana
Lawrence University
T-singular surfaces of general type. We explore the moduli space of stable surfaces, where the simplest of questions continue to remain open for almost all invariants. A few such questions: Of the allowable singularities, which ones actually occur on a stable surface? Which of these deform to smooth surfaces? How can we use this knowledge to find divisors in the moduli spaces? Can we develop a stratification of these moduli spaces by singularity type? Our focus will be on cyclic quotient singularities, with an emphasis on discussing concrete visual examples built out of rational, K3, and elliptic surfaces.
Tuesday, April 27, 2021
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Jayan Mukherjee
University of Kansas
Deformations of Galois Canonical Covers and Applications to Moduli of Surfaces with K^2=4p_g-8. In this talk we will deal with quadruple Galois canonical covers of surfaces of minimal degree. These surfaces satisfy K_X^2 = 4p_g(X)-8. The earlier case of double covers with K_X^2 = 2p_g(X)-4 was studied by Horikawa . This talk will concentrate on the irregular case. The classification of these surfaces by Gallego-Purnaprajna fall into four irreducible families and their work show that these surfaces behave like general surfaces of general type from various geometric perspectives. We show that except for one family, the general deformation of the canonical morphism is a morphism of degree two onto its image whose normalization is a ruled surface of appropriate genus. We further show that the canonical morphism of a general surface for each of these four families cannot be deformed into a finite birational morphism. As a consequence of our results, we show the existence of infinitely many irreducible uniruled components of the Gieseker moduli space containing surfaces with K_X^2 = 4p_g(X)-8, whose general element is a canonical double cover of a non-normal surface whose normalization is an elliptic ruled surface with invariant e = 0. A general surface of each of these moduli components is unobstructed although H^2(T_X) does not vanish. Our results are in sharp contrast with Horikawa's results on deformations of surfaces of general type with K_X^2 = 2p_g - 4. This is joint work with Purnaprajna Bangere, F.J. Gallego and D. Raychaudhury.
Tuesday, May 4, 2021
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Camilla Felisetti
Università di Trento
P=W conjectures for character varieties with a symplectic resolution. Character varieties parametrise representations of the fundamental group of a curve. In general these moduli spaces are singular, therefore it is customary to slightly change the moduli problem and consider smooth analogues, called twisted character varieties. In this setting, the P=W conjecture by de Cataldo, Hausel, and Migliorini suggests a surprising connection between the topology of Hitchin systems and Hodge theory of character varieties. In a joint work with M. Mauri we establish (and in some cases formulate) analogous P=W phenomena in the singular case . In particular we show that the P=W conjecture holds for character varieties which admit a symplectic resolution, namely in genus 1 and arbitrary rank and in genus 2 and rank 2. In the talk I will first mention basic notions of non abelian Hodge theory and introduce the P=W conjecture for smooth moduli spaces; then I will explain how to extend these phenomenas to the singular case, showing the proof our results in a specific example.
Tuesday, May 11, 2021
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Christopher Manon
University of Kentucky
When is a (projectivized) toric vector bundle a Mori dream space? Like toric varieties, toric vector bundles are a rich class of varieties which admit a combinatorial description. Following the classification due to Klyachko, a toric vector bundle is captured by a subspace arrangement decorated by toric data. This makes toric vector bundles an accessible test-bed for concepts from algebraic geometry. Along these lines, Hering, Payne, and Mustata asked if the projectivization of a toric vector bundle is always a Mori dream space. Süß and Hausen, and Gonzalez showed that the answer is "yes" for tangent bundles of smooth, projective toric varieties, and rank 2 vector bundles, respectively. Then Hering, Payne, Gonzalez, and Süß showed the answer in general must be "no" by constructing an elegant relationship between toric vector bundles and various blow-ups of projective space, in particular the blow-ups of general arrangements of points studied by Castravet, Tevelev and Mukai. In this talk I'll review some of these results, and then show a new description of toric vector bundles by tropical information. This description allows us to characterize the Mori dream space property in terms of tropical and algebraic data. I'll describe new examples and non-examples, and pose some questions. This is joint work with Kiumars Kaveh.
Tuesday, May 25, 2021
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Noble Williamson
UC Riverside
The search for effective divisor classes of M0,n. The Deligne-Mumford compactification of the moduli space of classes of genus zero smooth projective curves with n marked points is a very active object of study in algebraic geometry. One reason for this is that it has some interesting combinatorial structure, that provides abundant tools for its study. A fundamental question about it that remains unsolved is the structure of its cone of numerical equivalence classes of effective divisors. In particular, we want to know which effective divisors generate all others in the cone. We know that the divisors in the boundary are not enough so in this talk we will discuss some clever combinatorial strategies people have used to find non-boundary generating elements of the effective cone.
Tuesday, June 1, 2021
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)

Talk rescheduled.



Winter 2021

Date Speaker Title Abstract
Tuesday, January 19, 2021
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Luca Schaffler
KTH Royal Institute of Technology
Compactifications of moduli of points and lines in the projective plane. Projective duality identifies the moduli space Bn parametrizing configurations of n general points in the projective plane with X(3,n), parametrizing configurations of n general lines in the dual projective plane. When considering degenerations of such objects, it is interesting to compare different compactifications of the above moduli spaces. In this work, we consider Gerritzen-Piwek's compactification of Bn and Kapranov's Chow quotient compactification of X(3,n), and we show that they have isomorphic normalizations. We also construct an alternative compactification parametrizing all possible n-pointed central fibers of Mustafin joins associated to one-parameter degenerations of n points in the projective plane, which was proposed by Gerritzen and Piwek. We fully describe this alternative compactification for n=5,6. This is joint work with Jenia Tevelev.
Tuesday, January 26, 2021
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Changho Han
University of Georgia
Compact Moduli of lattice polarized K3 surfaces with nonsymplectic cyclic action of order 3. Observe that any construction of "meaningful" compactification of moduli spaces of objects involve enlarging the class of objects in consideration. For example, Deligne and Mumford introduced the notion of stable curves in order to compactify the moduli of smooth curves of genus g, and Satake used the periods from Hodge theory to compactify the same moduli space. After a brief review of the elliptic curve case (how those notions are the same), I will generalize into looking at various compactifications of Kondo's moduli space of lattice polarized K3 surfaces (which are of degree 6) with nonsymplectic Z/3Z group action; this involves periods and genus 4 curves by Kondo's birational period map in 2002. Then, I will extend Kondo's birational map to describe birational relations between different compactifications by using the slc compactifications (also known as KSBA compactifications) of moduli of surface pairs. The main advantage of this approach is that we obtain an explicit classification of degenerate K3 surfaces, which is used to find geometric meaning of points parametrized by Hodge-theoretic compactifications. This comes from joint works (in progress) with Valery Alexeev, Anand Deopurkar, and Philip Engel.
Tuesday, February 2, 2021
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Dustin Ross
San Francisco State University
Putting the volume back in volume polynomials. It is a strange and wonderful fact that Chow rings of matroids behave in many ways similarly to Chow rings of smooth projective varieties. Because of this, the Chow ring of a matroid is determined by a homogeneous polynomial called its volume polynomial, whose coefficients record the degrees of all possible top products of divisors. The use of the word "volume" is motivated by the fact that the volume polynomial of a smooth projective toric variety actually measures the volumes of certain polytopes associated to the variety. In the matroid setting, on the other hand, no such polytopes exist, and the goal of our work was to find more general polyhedral objects whose volume is measured by the volume polynomials of matroids. In this talk, I will motivate and describe these polyhedral objects. Parts of this work are joint with Jeshu Dastidar and Anastasia Nathanson.
Tuesday, February 9, 2021
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Nathan Ilten
Simon Fraser University
Fano schemes for complete intersections in toric varieties The study of the set of lines contained in a fixed hypersurface is classical: Cayley and Salmon showed in 1849 that a smooth cubic surface contains 27 lines, and Schubert showed in 1879 that a generic quintic threefold contains 2875 lines. More generally, the set of k-dimensional linear spaces contained in a fixed projective variety X itself is called the k-th Fano scheme of X. These Fano schemes have been studied extensively when X is a general hypersurface or complete intersection in projective space. In this talk, I will report on work with Tyler Kelly in which we study Fano schemes for hypersurfaces and complete intersections in projective toric varieties. In particular, I'll give criteria for the Fano schemes of generic complete intersections in a projective toric variety to be non-empty and of "expected dimension". Combined with some intersection theory, this can be used for enumerative problems, for example, to show that a general degree (3,3)-hypersurface in the Segre embedding of P^2×P^2 contains exactly 378 lines.
Tuesday, Febraury 16, 2021
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Brian Collier
UC Riverside
Global Slodowy slices for moduli spaces of Higgs bundles and holomorphic connections. Part 1. In this first talk I will introduce the moduli space of Higgs bundles on a projective curve and describe some of its properties and structures. I will then explain how the nonabelian Hodge correspondence topologically identifies the Higgs bundle moduli space and the moduli space of local systems and leads to a hyper-Kahler structure on the moduli space. Finally, we will describe how certain affine holomorphic Lagrangian subvarieties of these spaces are related to hyperbolic structures and complex projective structures on the topological surface.
Tuesday, February 23, 2021
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Brian Collier
UC Riverside
Global Slodowy slices for moduli spaces of Higgs bundles and holomorphic connections. Part 2. In this talk, we will discuss the relationship between the Hitchin section in the Higgs bundle moduli space and Beilinson-Drinfeld's parameterizations of space of opers. This generalizes our previous discussion on hyperbolic and projective structures. In these theories, a key role is played by a principal nilpotent and the Borel subgroup. In the remaining time, we will discuss joint work with Andrew Sanders which generalizes this story to arbitrary parabolics. Time permitting, we will discuss how the objects we parameterize are related to higher Teichmüller theory and also mention some open problems.
Tuesday, March 9, 2021
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Matthew Satriano
University of Waterloo
New types of heights with connections to the Batyrev-Manin and Malle Conjectures The Batyrev-Manin conjecture gives a prediction for the asymptotic growth rate of rational points on varieties over number fields when we order the points by height. The Malle conjecture predicts the asymptotic growth rate for number fields of degree d when they are ordered by discriminant. The two conjectures have the same form and it is natural to ask if they are, in fact, one and the same. We develop a theory of point counts on stacks and give a conjecture for their growth rate which specializes to the two aforementioned conjectures. This is joint work with Jordan Ellenberg and David Zureick-Brown. No prior knowledge of stacks will be assumed for this talk.



Fall 2020

 
Date Speaker Title Abstract
Tuesday, October 13, 2020
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Patricio Gallardo
UC Riverside
On wonderful blow-ups. Techniques for constructing good compactifications of an open set is one of the main problems within Algebraic Geometry. In this talk, I will describe a tool known as Wonderful Compactifications due to Li-Li which generalizes earlier work by De Concini-Procesi on hyperplane arrangements. Applications to moduli problems will be described as well.
Tuesday, October 20, 2020
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Patricio Gallardo
UC Riverside
On wonderful blow-ups (second part). We will continue describing the theory of wonderful blow-ups. A particular focus is given to applications within moduli theory as well as open problems.
Tuesday, October 27, 2020
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Benjamin Schmidt
Leibniz Universität Hannover
A curious moduli space on cubic threefolds. The intermediate Jacobian J(X) of a cubic threefold X was introduced by Clemens and Griffiths in 1972 to prove irrationality of cubic threefolds. It is an abelian variety that can be thought of as parametrizing degree zero cycles in dimension one up to rational equivalence. In this talk we will concentrate on its theta divisor ϴ. Clemens and Griffiths proved the so-called Torelli theorem for cubic threefolds that says that the pair (J(X),ϴ) determines the cubic threefold. Shortly after, Mumford pointed out that X can be recovered just from the singularities of the theta divisor. In fact, it has a unique singularity whose tangent cone is the affine cone of the cubic X. Blowing the singularity up yields a resolution of singularities. We will construct this resolution as a moduli space of rank three vector bundles. This allows us to recover the so-called derived Torelli theorem. It roughly says that a certain triangulated subcategory (called the Kuznetsov component of X) of the bounded derived category of coherent sheaves determines X.
Tuesday, November 3, 2020
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
US Election Day
Election Day. No seminar meeting on US Election Day.
Tuesday, November 10, 2020
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Olivia Dumitrescu
University of North Carolina at Chapel Hill
Lagrangian correspondence between Hitchin and de Rham moduli spaces. In 2008 Simpson conjectures the existence of a holomorphic Lagrangian foliation in the de Rham moduli space of holomorphic G-connections for a complex reductive group G. The purpose of the talk is to establish the existence of a holomorphic Lagrangian foliation in the de Rham moduli space of holomorphic SL_2(C)-connections defined on a smooth connected projective curve C of genus at least 2. The conjectural holomorphic Lagrangian foliation does not seem to constitute a holomorphic Lagrangian fibration. I will present an algebraic geometry description of the Lagrangian correspondence of conformal limits, based on the work of Simpson. This talk is based on joint work with Jennifer Brown and Motohico Mulase.
Tuesday, November 17, 2020
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Dagan Karp
Harvey Mudd College
The Chow ring of heavy/light Hassett spaces via tropical geometry. Hassett spaces in genus 0 are moduli spaces of weighted pointed stable rational curves; they are important in the minimal model program and enumerative geometry. We compute the Chow ring of heavy/light Hassett spaces. The computation involves intersection theory on the toric variety corresponding to a graphic matroid, and rests upon the work of Cavalieri-Hampe-Markwig-Ranganathan. This is joint work with Siddarth Kannan and Shiyue Li.    Slides
Tuesday, November 24, 2020
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Antonio Laface.
University of Concepcion
Blown-up toric surfaces with non-polyhedral effective cone. I will discuss examples of projective toric surfaces whose blow-up at a general point has a non-polyhedral pseudoeffective cone, both in characteristic 0 and in positive characteristic. As a consequence, the pseudo-effective cone of the Grothendieck-Knudsen moduli space M0,n is not polyhedral for n ≥ 10 in characteristic 0 and for an infinite set of primes of positive density in positive characteristic.
Tuesday, December 1, 2020
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Javier Gonzalez-Anaya
UC Riverside
Negative curves in blowups of weighted projective planes. Blowups of toric varieties at general points have played a central role in many recent developments concerning the birational geometry of some moduli spaces. As part of this ongoing program, we'll discuss what is known about the Mori dream space property for blowups of weighted projective planes at a general point. By a result of Cutkosky, such a variety is a Mori dream space if and only if it contains two non-exceptional irreducible curves disjoint from each other; one of them having non-positive self-intersection. Such a curve a is called a “negative curve”. Negative curves largely govern the Mori dream space property for these varieties. In this talk I will survey what is currently known about their existence, how they "interact" with each other and how these interactions inform us about the Mori dream space property in many cases.
Tuesday, December 8, 2020
Start: 11:00 AM
Location: Online via Zoom (Please email the organizers if interested.)
Bernt Ivar Utstøl Nødland
Norwegian Defence Research Establishment
Cox rings of projectivized toric vector bundles. A toric vector bundle is a torus equivariant vector bundle on a toric variety. To a toric vector bundle one can associate a collection of lattice polytopes called the parliament of polytopes. We show how we can use these polytopes to give a description of the Cox ring of a projectivized toric vector bundle.