BiMs: Bielefeld-Münster Seminar
on
Groups, Geometry and Topology

Friday, February 06, 2026

Organizers: Barbara Baumeister, Kai-Uwe Bux, Linus Kramer

10:15 - 11:15 Noam von Rotberg

Title: Enumerating representations by extending matrices

Representation zeta functions of groups enumerate a groups' irreducible complex representations of each finite degree. Computing these zeta functions for nilpotent groups can be achieved by enumerating skew-symmetric matrices over finite Artinian rings with given elementary divisor type.
We are interested in understanding how the representation zeta function changes under taking relative-free products with Z^n. On the level of matrices this corresponds to counting how a skew symmetric matrix can be extended by a generic row and column to obtain a prescribed elementary divisor type.

11:15 - 11:45 Coffee break

11:45 - 12:45

Anna Cascioli

Title: C*-simplicity and dynamics on the space of amenable subgroups

Abstract: C*-simplicity is a property of groups that naturally arises in the study of their unitary representations and has recently been explored through group-theoretic and dynamical approaches. In this talk, I will focus on characterizations of C*-simplicity via the action of the group on the space of its amenable subgroups, and how measure-theoretic properties of this action allow us to distinguish different classes of C*-simple groups. This is work in progress with Martín Gilabert Vio and Eduardo Silva.

14:00 - 15:00 Georges Neaime

Title: Non-crossing partitions for exceptional hereditary curves

Abstract: The talk is about a recent preprint (https://arxiv.org/abs/2512.01729), where I will focus on aspects related to group theory and geometric group theory.

15:00 - 15:30 Coffee break

15:30 - 16:30 Sira Busch

Icosahedral buildings.

In 'A local approach to buildings', Jacques Tits showed that the geometry associated with a spherical Coxeter diagram distinct from H_3 and H_4 is a building if and only if it satisfies a certain condition (Int) concerning the intersection of the shadows of two elements.
In this talk, we introduce a simple point–line axiom system for buildings of type H_3 and show that (Int) implies all of these axioms. We then give a new elementary geometric proof of the non-existence of thick buildings of types H_3 and H_4.

Supported by Mathematics Münster and TRR 358 Integral Structures and Representation Theory.