A dichotomy in dimension four
Does every four-manifold admit either no smooth structure or infinitely many of them? I'll report on recent work, joint with A. Stipsicz and Z. Szabó, addressing this question for four-manifolds with finite cyclic fundamental groups. A bonus discussion may feature fake projective planes in the context of yet another dichotomy.
tba
Group rings and hyperbolic geometry
In the 60's Cohn showed that all ideals in the group algebra of a free group are free. Bass and Wall used this result to show that all two-dimensional complexes with free fundamental groups are standard: they are all homotopy equivalent to wedges of circles and 2-spheres. The goal of this talk is to describe recent results of this type for groups acting on hyperbolic spaces. I will discuss an algorithm showing that in the group algebra of a group acting on a hyperbolic space, ideals generated by ``few'' elements are free (where "few'' is a function of the minimal displacement of the action) and an application to complexity of cell decompositions of hyperbolic manifolds. Joint work with Thomas Delzant.
Models for Differentiable Stack Cohomology - A journey via equivariant cohomology and Lie groupoids
Analytic Rings
Analytic prismatization
MPI-Oberseminar jointly with "Arithmetic Geometry, A conference in Honor of Hèléne Esnault on the occasion of Her 70th Birthday" at IHES:
https://indico.math.cnrs.fr/event/11114/
Abstract: Prismatic cohomology is a unifying p-adic cohomology of p-adic formal schemes. Motivated by questions on locally analytic representations of p-adic groups and the p-adic Simpson correspondence, an extension of prismatic cohomology to rigid-analytic spaces (over Q_p or over F_p((t))) has been sought. We will explain what form this should take, and our progress on realizing this picture. This includes a degeneration from the analytic Hodge-Tate stack underlying the p-adic Simpson correspondence to a similar (analytic) stack related to the Ogus-Vologodsky correspondence in characteristic p. This is joint work in progress with Johannes Anschütz, Arthur-César le Bras and Juan Esteban Rodriguez Camargo.
Hecke eigen-functions for curves over local non-archimedian fields
We give a survey of some recent constrictions and results concerning Hecke operators for the moduli space of G-bundles on a smooth projective curve X defined over a local non-archimedian field F (possibly with level structures). These operators are somewhat analogous to the usual Hecke operators in the case when F is a finite field but describing their common eigen-values is a much more difficult task. We'll discuss on what space the Hecke operators act, formulate some general conjectures about eigen-functions and consider some examples. In the end (if time permits) I would like to discuss the example when X is of genus 0, and we study bundles with triviliazation at two points. This example is closely related to the usual representation theory of G(F).
Classical real local Langlands for GL_2(R)
Computing differentiable stack cohomology via multiplicative Ehresmann connections
tba
Lifting differential equations
I will start by discussing 'frequently occurring congruences' and the related notion of congruence sheaves. I will then introduce
a method to construct lifts of differential equations by using generalized Taylor's formula in a systematic way.
The talk is based on joint work with Ilia Gaiur.
tba *Please note location!*[Bonn symplectic geometry seminar]
Stability problems and differential graded Lie algebras
Stability problems appear in various forms throughout geometry and algebra. In differential geometry, given a vector field on a manifold that vanishes in a point, one can ask when nearby vector fields also vanish in a point. In algebra, given a Lie algebra and a subalgebra, one can ask when all deformations of the ambient Lie algebra also admit a Lie subalgebra of the same dimension. I will show that both questions are instances of a general question about differential graded Lie algebras. Under a finite-dimensionality assumption which is satisfied in the examples above, I will give a sufficient condition for a positive answer to the general question. I will then discuss some applications.
Proper sheaves on Bun_G
IMPRS seminar on various topics: single talk
On the distribution of supersingular primes for abelian surfaces
In 1976, Lang and Trotter conjectured the asymptotic growth for the number of primes p up to x for which the reduction of a non-CM elliptic curve E/Q at p is supersingular. Though the conjecture is still open, we now have unconditional upper and lower bounds thanks to the work of several mathematicians. However, less has been studied for the distribution of supersingular primes for abelian surfaces (even conjecturally). In this talk, I will present a recent work on unconditional upper bounds for the number of primes p up to x, for which the reduction of a fixed abelian surface at p is supersingular.
The equation $c_1x_1^k+...+c_sx_s^k=0$
tba [OS Differentialgeometrie]
Teleparallel Newton—Cartan Gravity
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