Abstracts

Week of April 13, 2025

April 14, 2025
Group Actions and Dynamics Properness criteria for affine actions of Anosov groups 4:00pm -
KT 207

I will present some criteria (necessary or sufficient) for the action on the affine space of a group Gamma of affine transformations to be proper. This is joint work with Fanny Kassel.

The main of these criteria links properness of action to the divergence of a parameter called the Margulis invariant. This invariant measures roughly the translation part of an affine transformation, but in a way that is invariant by conjugation.

This link was already known in some special cases (and has often been exploited to construct proper actions). We tried to establish it in as general setting as possible. We proved it in particular if Gamma has some suitable Anosov property (with respect to some natural parabolic subgroup, that depends on the affine group we are working in).

I will possibly also evoke some other invariants similar to the Margulis invariant, that could lead to criteria that work in even more general settings.

Analysis Spectral theory of high-contrast random media 4:00pm -
KT 205

The talk is concerned with the rigorous mathematical description of propagation and localisation of waves in a particular class of composite materials with random microscopic geometry, called micro-resonant (or high-contrast) random media: small inclusions of a “soft” material are randomly dispersed in a “stiff” matrix.  The highly contrasting physical properties of the two constituents,  combined with a particular scaling of the inclusions,  result in microscopic resonances, which manifest macroscopically by allowing propagation of waves in the material only within certain ranges of frequencies (band-gap spectrum).

High-contrast media with periodically distributed inclusions have been extensively studied and numerous results are available in the literature.  However,  their stochastic counterparts, which model more realistic scenarios and may exhibit localisation,  are far from being well understood from a mathematical viewpoint.  In my talk I will give an overview of existing results through the prism of stochastic homogenisation and spectral theory,  and discuss recent advances and ongoing work.

Based on joint works with M. Cherdantsev, I. Velčić, P. Bella and M. Täufer.

April 15, 2025
Geometry & Topology A ``cubist'' decomposition of the Handel-Mosher axis bundle & the conjugacy problem for Out(F_r) 4:00pm -
KT 207

Outer automorphisms of free groups are largely studied via their action on Culler-Vogtmann Outer space. However, unlike in hyperbolic space or Teichmuller space (surface) settings, the dynamically minimal (fully irreducible) free group outer automorphisms act on Culler-Vogtmann Outer space with a collection of axes, whose closure is the Handel-Mosher ``axis bundle.'' Not much of the structure of this axis bundle has yet been understood. Together with Chi Cheuk Tsang, we prove that the axis bundle has a "cubist" structure and use this structure to find preferred axes for these outer automorphisms. We then use these axes to provide a solution to the fully irreducible conjugacy problem. This work can be seen as in analogy with that of Hamenstadt and Agol in the surface setting.

April 16, 2025
Robinson Lectures Bass-Note Spectra of locally uniform geometries 4:00pm -

We formulate and report on the problem of the Bass-Note 

Spectrum of an invariant operator as one varies over locally uniform 

geometries. In the Euclidean setting this recasts classical problems of

Mahler from the geometry of numbers in a new light. For certain operators 

homogeneous dynamics can be used decisively. In the non-Euclidea setting 

of hyperbolic manifolds we review some recent developments using the

conformal bootstrap method and of random covers to study the Bass- Note

spectra. We highlight the theme and impact of rigidity.

April 17, 2025
Quantum Topology and Field Theory TBA 4:30pm -
KT 801
April 18, 2025
Friday Morning Seminar TBA 10:00am -
KT 801

A relaxed-pace seminar on impromptu subjects related to the interests of the audience. Everyone is welcome. The subjects are geometry, probability, combinatorics, dynamics, and more!

Algebra and Geometry lecture series Vertex algebra and moduli of Higgs bundles III 3:00pm -
KT801

Sam will discuss the connection between VOA and the cohomology of the Hitchin moduli space, as a continuation of his first 2 lectures.