UBC Algebraic Geometry Seminar

Time and place: Monday, 3-4 pm, MATH 126

Organizers: Kai Behrend, Jim Bryan and Shubhodip Mondal

Please click on the title to see the abstract

Winter Term 2, 2025

Date Speaker Title
Jan 6, 2025 Lucy Yang (Columbia)
Involutions and the Brauer group in derived algebraic geometryClassical results of Albert and Saltman (extended by Knus-Parimala-Srinivas) have established a connection between the existence of (anti-)involutions on the central simple algebras A used to define the Brauer group and 2-torsion Brauer classes. Moreover, the presence of more general forms of involutions is related to the behavior of Brauer classes under corestriction along quadratic extensions. In this work, we introduce and study a generalization of these ideas to derived algebraic geometry. We investigate how the data of an involution on A is reflected in additional structure on its category of modules. Using the theory of Poincare infinity-categories developed by Calmes-Dotto-Harpaz-Hebestreit-Land-Moi-Nardin-Nikolaus-Steimle, we introduce involutive versions of the Picard and Brauer group and relate them to their non-involutive counterparts.
Jan 13, 2025 Sanath Devalapurkar (Harvard)
Hodge theory in characteristic p and homotopy theoryAlgebraic topology provides many useful invariants of topological spaces, like singular cohomology. If our topological space is a smooth manifold, then one can compute singular cohomology via differential forms. If our topological space is furthermore a complex algebraic variety, then its cohomology can furthermore be computed via harmonic forms: this is the main result of Hodge theory. Although it is typically defined analytically, one can nevertheless make sense of the Hodge filtration on (de Rham) cohomology for algebraic varieties X defined over the finite field F_p. A beautiful result of Deligne and Illusie states that Hodge theory continues to work if dim(X) \leq p and the defining equations for X can be lifted to the (p-adic) integers Z. They asked the question of what happens if dim(X) > p. In this talk, I will explain how one can use tools from homotopy theory, like topological Hochschild homology and complex cobordism, to give conditions on X under which Hodge theory continues to work when dim(X) \leq p^n for some n > 0. Time permitting, I will explain how this relates to recent work of Bhatt-Lurie and Drinfeld on "prismatic cohomology".
Jan 20, 2025 Igor Rapinchuk (Michigan State University)
Groups with good reduction and buildings Over the last few years, the analysis of algebraic groups with good reduction has come to the forefront in the emerging arithmetic theory of algebraic groups over higher-dimensional fields. Current efforts are focused on finiteness conjectures for forms of reductive algebraic groups with good reduction that share some similarities with the famous Shafarevich Conjecture in the study of abelian varieties. Most results on these conjectures obtained so far have ultimately relied on finiteness properties of appropriate unramified cohomology groups. However, quite recently, methods based on building-theoretic techniques have emerged as a promising alternative approach. I will showcase some of these developments by sketching a new proof of a theorem of Raghunathan-Ramanathan concerning torsors over the affine line.
Jan 27, 2025 Sebastien Picard (UBC)
Calabi-Yau threefolds crossing nodal singularitiesWe will review the conifold transition, which is a topological surgery for Calabi-Yau threefolds. The process involves a deformation of complex structure and a small resolution of nodal singularities. We discuss the geometrization of this process and its study by complex analytic and differential geometric methods. This is joint work with T. Collins and S.-T. Yau.
Feb 3, 2025 Cancelled
Feb 10, 2025 Federico Scavia (CNRS and Universite Sorbonne Paris Nord)
Galois representations modulo p that do not lift modulo p^2 For every finite group H and every finite H-module A, we determine the subgroup of negligible classes in H^2(H,A), in the sense of Serre, over fields with enough roots of unity. As a consequence, we show that for every odd prime p and every field F containing a primitive p-th root of unity, there exists a continuous 3-dimensional mod p representation of the absolute Galois group of F(x_1,...,x_p) which does not lift modulo p^2. We also construct continuous 5-dimensional Galois representations mod 2 which do not lift modulo 4. This answers a question of Khare and Serre, and disproves a conjecture of Florence. This is joint work with Alexander Merkurjev.
Feb 24, 2025 Sabin Cautis (UBC)
Abelian Hall categoriesWe will explain how, to any quiver, one can associate a finite length abelian category which categorifies the corresponding K-theoretic Hall algebra. The simples in this category provide a (dual) canonical basis of the Hall algebra. In particular, if the quiver is affine, this provides a basis for the positive half of the corresponding quantum toroidal algebra. We also explain how this abelian category is naturally endowed with renormalized r-matrices.
March 3, 2025 Stephen Pietromonaco (Michigan)
Curve Counting on Abelian Surface Fibrations and Quasi-Siegel Modular Forms A big conjecture which emerged from the math-physics interface is that the Gromov-Witten potentials of a Calabi-Yau threefold are generalized quasi-modular objects. In on-going work, joint with Aaron Pixton, we study the case of Picard rank 3 Abelian surface fibrations, where quasi-modular forms of Siegel type are expected to arise. We focus on the example of the banana manifold where we prove that the Gromov-Witten potentials satisfy the elliptic transformation law of a Siegel-Jacobi form for the E_{8} lattice. Moreover, a conjectural form is given in genus 0, and degree 1 over the base of the Abelian surface fibration.
March 10, 2025 Ishai Dan-Cohen (Ben Gurion University)
A motivic Weil height machine for curvesThe rational points of a hyperbolic curve over a number field map to the set of augmentations of the associated motivic algebra. An expectation, closely related to the Grothendieck section conjecture, is that the set of augmentations which are locally geometric is equal to the set of rational points. We provide evidence for this expectation by extending aspects of the "Weil height machine" to the set of locally geometric augmentations. As an application, we obtain a finiteness result which extends the classical Manin--Demjanenko theorem. This is ongoing joint work with L. Alexander Betts.
March 17, 2025 Emanuel Reinecke (IHES)
Relative Poincare duality in nonarchimedean geometryWhile the etale cohomology of Z/p-local systems on smooth p-adic rigid spaces is in general hard to control, it becomes more tractable when the spaces are proper. For example, in the proper case it is finite-dimensional and has recently been shown in work of Zavyalov and of Mann to satisfy Poincare duality. In my talk, I will explain a new, essentially "diagrammatic" proof of Poincare duality in this context, which also works for more general spaces and coefficients and in the relative setting. The argument relies on a novel construction of trace maps for smooth morphisms of rigid spaces. Joint work with Shizhang Li and Bogdan Zavyalov.
March 24, 2025 Shubhodip Mondal (UBC)
Zeta functions of algebraic varieties and special valuesIn 1966, Tate proposed the Artin-Tate conjectures, which expresses special values of zeta function associated to surfaces over finite fields. Conditional on the Tate conjecture, Milne-Ramachandran formulated and proved similar conjectures for smooth proper schemes over finite fields. The formulation of these conjectures already relied on other unproven conjectures. In this talk, I will discuss an unconditional formulation and proof of these conjectures. A key new ingredient is the notion of "stable Bockstein characteristic" that we will introduce.
March 31, 2025
April 7, 2025

Winter Term 1, 2024

Date Speaker Title
Sept 9, 2024 Organizers
Organizational meeting.

We will meet to discuss the schedule for the semester.

Sept 16, 2024 Will Donovan (YMSC)
Derived symmetries for crepant resolutions of hypersurfaces

Given a singularity with a crepant resolution, a symmetry of the derived category of coherent sheaves on the resolution may often be constructed using the formalism of spherical functors. I will introduce this, and discuss work in progress on general constructions of such symmetries for hypersurface singularities. This builds on previous results with Segal, and is inspired by work of Bodzenta-Bondal.

Sept 23, 2024 Felix Thimm (UBC)
Refined Counting of Colored Plane Partitions

Degree 0 Donaldson-Thomas (DT) invariants of certain orbifolds correspond to counts of colored plane partitions, which are configurations of boxes in 3D space, where the boxes are colored in a way determined by the orbifold structure. We will explain this correspondence and compute equivariant K-theoretically refined degree 0 orbifold DT invariants for some orbifolds. The computation combines a technique called factorization with more classical combinatorial arguments.

Oct 7, 2024 Sebastian Gant (UBC)
Splitting stably free modules

We study the question of when a generic stably free module splits off a free summand of a given rank. This question has the following geometric interpretation due to M. Raynaud. Let V(r,n) denote the Stiefel variety GL(n)/GL(n-r) over a field k. There is a projection map V(r,n) -> V(1,n) given by “forgetting frames.” Raynaud showed that V(r,n) -> V(1,n) has a section if and only if the following holds: if P is any module over any k-algebra R with the property that P+R is isomorphic to R^n, then P has a free factor of rank r-1. Using machinery from A1-homotopy theory, we characterize those n for which the map V(r,n) -> V(1,n) has a section in the cases r <= 4 and under some restrictions on the base field. This is joint work with Ben Williams.

Oct 21, 2024 Sujatha Ramdorai (UBC)
Growth of Mordell-Weil ranks of elliptic curves

Let E be an elliptic curve over a number field F. We will discuss the structure of certain modules that arise in the Iwasawa theory of elliptic curves and their applications to the growth of Mordell-Weil ranks along infinite Galois extensions of F with noncommutative Galois groups.

Oct 28, 2024 Balazs Elek (UBC)
Affine Kazhdan-Lusztig varieties and Grobner bases

A Kazhdan-Lusztig variety is the intersection of a Schubert variety with an affine cell in a flag manifold. Therefore, one can obtain local equations for Schubert varieties by using coordinates on the affine cell. Building on the work of Fulton and Knutson/Miller, in finite type A, Woo and Yong gave a Grobner basis for Kazhdan-Lusztig ideals. We will describe a generalization of their result to the affine type A flag manifold. We will define linear charts on affine flag manifolds using Bott-Samelson varieties, describe an analogue of Fulton's essential set, then use a result of Knutson on geometric vertex decompositions to show that our equations give a Grobner basis. This is joint work with Daoji Huang.

Nov 4, 2024 Jerry Yu Fu (Caltech)
The p-adic analog of the Hecke orbit conjecture and density theorems toward the p-adic monodromy

The Hecke orbit conjecture predicts that Hecke symmetries characterize the central foliation on Shimura varieties over an algebraically closed field $k$ of characteristic $p$. The original conjecture predicts that on the $\bmod p$ reduction of a Shimura variety, any prime-to- $p$ Hecke orbit is dense in the central leaf containing it, and was recently proved by a series of nice papers. However, the behavior of Hecke correspondences induced by isogenies between abelian varieties in characteristic $p$ and $p$-adically is significantly different from the behavior in characteristic zero and under the topology induced by Archimedean valuations. In this talk, we will formulate a p-adic analog of the Hecke orbit conjecture and investigate the $p$-adic monodromy of $p$-adic Galois representations attached to points of Shimura varieties of Hodge type. We prove a density theorem for the locus of formal neighborhood associated to the mod $p$ points of the Shimura variety whose monodromy is large and use it to deduce the non-where density of Hecke orbits under certain circumstances.

Nov 18, 2024 Jonathan Yang (UBC)
Mixed Volumes of Matroids

Matroids are combinatorial objects that abstract the notion of independence. The motivating examples are matroids arising from vector configurations. An important tool for studying matroids is the associated Chow ring. A recent result of Adiprasito, Huh, and Katz is that Chow rings arising from matroids behave "nicely" in ways similar to cohomology rings of smooth projective varieties. This result has geometric inspiration, a combinatorial proof, and powerful consequences in matroid theory. In this talk, we will introduce this geometrically inspired approach to matroid theory. Then, we will present a new computation for the "mixed volume of a matroid", that is, the matroid analog to the integration map on smooth projective varieties. This is joint work with Andy Hsiao, and Kalle Karu.

Nov 25, 2024 Arnab Kundu (Toronto)
Motivic cohomology in mixed-characteristic

Motivic cohomology is a cohomology theory that can be defined internally within Grothendieck's category of motives. Voevodsky developed this theory for smooth varieties, demonstrating its profound connections to algebraic cycles and algebraic K-theory. However, its behaviour beyond the smooth case remains less well understood. Building upon recent advancements by Bachmann, Elmanto, Morrow, and Bouis, we establish its A^1-homotopty invariance for a broader class of "smooth" schemes. This is part of ongoing work in collaboration with Tess Bouis.

Dec 2, 2024 Cancelled
Dec 9, 2024 Amin Soofiani (UBC)
Hensel's lemma for the norm principle for groups of type D_n

Given a finite separable field extension and a linear algebraic group defined over the base field, we can study the "Norm Principle", which examines how the base change of the group behaves with respect to the norm map of the field extension. It remains an open question whether the norm principle holds for all linear algebraic groups. In this talk, we will recall a Galois cohomology approach to this problem, and discuss the norm principle for groups of type D_n, in particular over complete discretely valued fields.

From Sept 1, 2024 to Aug 31, 2025, the UBC AG seminar is partially funded by PIMS.

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