Tag - Topology

Mohammed Abouzaid: Theory of bordisms

6th August, 2024

In this introductory lecture, which should be accessible to a general mathematical audience, I will review the classical bordism theory of manifolds, from its origin in Poincare's work, to the subsequent development by Pontryagin, Thom, Milnor, Wall, and Quillen among others.

Lecture 2: Bordism of orbifolds

An orbifold is a space with additional structure that describes it locally as the quotient of a manifold by a finite group. I will describe Pardon's recent result which reduces the study of orbifolds to the study of manifolds with Lie group actions. Then I will explain the relationship between equivariant and orbifold bordism, and formulation some structural properties of this theory.

Lecture 3: Bordism of derived orbifolds

The notion of a derived orbifold arises naturally in pseudo-holomorphic curve theory, and plays a central role in the emerging field of Floer homotopy. I will explain how it is related to the notion of "homotopical bordism" due to tom Dieck in the 1970s, and formulate some conjectures about its structure in the complex oriented case.

Marc Stephan: An equivariant BGG correspondence and applications to free A4-actions

13th June, 2024

A classical question in the theory of transformation groups asks which finite groups can act freely on a product of spheres. For instance, Oliver showed that the alternating group A4 can not act freely on any product of two equidimensional spheres.

I will report on joint projects with Henrik Rüping and Ergün Yalcin and explain that for 'most' dimensions m and n, there is no free A4-action on Sm × Sn and whenever there exists such a free action, then the corresponding cochain complex with mod 2 coefficients is rigid: its equivariant homotopy type only depends on m and n.

This involves an equivariant extension of Carlsson’s BGG correspondence in order to classify perfect complexes over 𝔽2[A4] with 4-dimensional total homology.

Matthew Morrow: Algebraic K-theory and p-adic arithmetic geometry

22nd April, 2024

To any unital, associative ring R one may associate a family of invariants known as its algebraic K-groups. Although they are essentially constructed out of simple linear algebra data over the ring, they see an extraordinary range of information: depending on the ring, its K-groups can be related to zeta functions, corbordisms, algebraic cycles and the Hodge conjecture, elliptic operators, Grothendieck's theory of motives, and so on.

Our understanding of algebraic K-groups, at least as far as they appear in algebraic and arithmetic geometry, has rapidly improved in the past few years. This talk will present some of the fundamentals of the subject and explain why K-groups are related to the ongoing special year in p-adic arithmetic geometry. The intended audience is non-specialists.

Paul Minter: An Overview of Geometric Measure Theory, Area Minimising Currents, and Recent Progress

8th April, 2024

Structures which minimise area appear in numerous geometric contexts often related to degeneration phenomena. In turn, in many situations these structures also reflect the ambient geometry in some way (they are ‘calibrated’) and so they may provide a way to study the interplay between geometry and topology, as has historically been the case for variational methods in geometry.

Almgren developed a theory which established that these area minimising structures are manifolds away from a codimension 2 ‘singular set’. The singular set itself, however, remained rather mysterious, including whether it necessarily has locally finite measure, unique tangent cones, or geometric structure (rectifiability).

In this talk I will attempt to give an overview of these ideas, as well as of recent work (joint with Camillo De Lellis and Anna Skorobogatova) answering some of the questions above related to singularities of area minimizers.

Maxime Ramzi: Categorifying spectra and the theorem of the heart

28th March, 2024

The goal of this talk will be to present the results from my recent joint work with Vova Sosnilo and Christoph Winges, where we prove that every spectrum is the (non-connective) K-theory spectrum of a stable category. Our main application of this is the disproof of a conjecture by Antieau-Gepner-Heller about a non-connective version of the theorem of the heart in the non-noetherian setting; but I will also try to mention other perspectives on this result.

Maxime Ramzi: On Endomorphisms of THH

27th March, 2024

Topological Hochschild homology is an important invariant, closely related to algebraic K-theory, and can be seen as a non-commutative analogue of de Rham chains.

In this talk, I will describe various computations of the ring/monoid of endomorphisms of THH in different variants, with an approach based on a generalized version of the Dundas-McCarthy theorem.

Oscar Randal-Williams: Cohomology of moduli spaces: a case study

25th March, 2024

I will explain recent work of Bergström–Diaconu–Petersen–Westerland, and of Miller-Patzt-Petersen-R-W, which uses methods which have been developed over the last 25 years for studying the topology of certain moduli spaces in order to answer a question in arithmetic statistics (the function field analogue of a conjecture of Conrey-Farmer-Keating-Rubinstein-Snaith on moments of quadratic L-functions). My focus will be on the translation of this question to a problem in topology, and some of the modern methods which go into solving this problem.

Jay Shah: Real topological Hochschild homology, C2-stable trace theories, and Poincaré cyclic graphs

29th February, 2024

To study topological Hochschild homology as an invariant of stable ∞-categories and endow it with its universal property in this context, Nikolaus introduced the formalism of stable cyclic graphs and trace theories (after Kaledin). On the other hand, Poincaré ∞-categories are a C2-refinement of stable ∞-categories that provide an adequate formalism for studying real and hermitian algebraic K-theory, which should be then well-approximated by the real cyclotomic trace. In this talk, we explain how to systematically provide Poincaré refinements of all the components of Nikolaus's approach to stable trace theories.

Tess Bouis: Motivic Cohomology of Mixed Characteristic Schemes

14th February, 2024

I will present a new theory of motivic cohomology for general (qcqs) schemes. It is related to non-connective algebraic K-theory via an Atiyah-Hirzebruch spectral sequence. In particular, it is non-A1-invariant in general, but it recovers classical motivic cohomology on smooth schemes over a Dedekind domain after A1-localisation. The construction relies on the syntomic cohomology of Bhatt-Morrow-Scholze and the cdh-local motivic cohomology of Bachmann-Elmanto-Morrow, and generalises the construction of Elmanto-Morrow in the case of schemes over a field.

Amie Wilkinson: Dynamical Asymmetry Is C1-Typical

5th February, 2024

I will discuss a result with Bonatti and Crovisier from 2009 showing that the C1 generic diffeomorphism f of a closed manifold has trivial centralizer; i.e., fg = gf implies that g is a power of f. I'll discuss features of the C1 topology that enable our proof (the analogous statement is open in general in the Cr topology, for r > 1). I'll also discuss some features of the proof and some recent work, joint with Danijela Damjanovic and Disheng Xu that attempts to tackle the non-generic case.