This is a series of one-day meetings, with the aim of bringing together geometric group theorists, topologists, and geometers in the North-Rhine Westphalia region of Germany. Sign up to the mailing list here.
5th Meeting - Bielefeld, 25th June 2026
Local organizers: Martina Conte and Georges Neaime.
Location and directions:
This meeting takes place at Bielefeld University. All talks are scheduled in rooms V3-201 and V2-210/216. For precise location and travel directions, please click HERE.
Registration:
No registration is needed to participate in this event. Just for this edition, if you are interested in joining the dinner, please send an email to one of the local organisers for planning purposes.
Further information:
On June 26th we are hosting a satellite event of this GGNRW meeting at Bielefeld University, for which registration is required.
Please click HERE to visit the website, where you can find further information and instructions on how to register and apply for funding. There will also be the possibility for participants to present a poster or give a short talk.
Schedule:
12:30, Room V3-201, Arrival
13:00-14:00, Room V3-201, Waltraud Lederle (Bielefeld University)
Compact invariant random subgroups
An IRS on a locally compact group is a conjugacy-invariant probability measure on the space of its subgroups; a way to study stabilizers of probability measure preserving actions. We are interested in those IRS that give full measure to the set of compact subgroups. This talk is about what we know about those, how it connects to the structure theory of locally compact groups, and what we would still like to figure out. Joint with Tal Cohen, Helge Glöckner and Gil Goffer.
14:00-14:30, Room V3-201, Conference Picture & Coffee Break
14:30-15:30, Room V3-201, Eduard Schesler (Karlsruhe Institute of Technology)
On the flexible geometry of Thompson's group V
A finitely generated group G is said to have uniform exponential growth if the number of elements in G that are represented by words of length at most n over some generating set of G is bounded below by a single exponential function in n that does not depend on the generating set. In 1981, Gromov asked whether every group of exponential growth has uniformly exponential growth. While this has been confirmed for many natural classes of groups, the general answer turned out to be negative, as shown by Wilson in 2004. Since the groups constructed by Wilson are not finitely presented, the question remained open whether there exist finitely presented groups of non-uniform exponential growth. In this talk I will show that this is indeed the case, by proving that Thompson's group V has non-uniform exponential growth. This talk is based on joint work with Roman Sauer.
15:30-16:00, Coffee Break
16:00-17:00, Room V2-210/216, Giles Gardam (University of Bonn)
Tame groups and wild rings
Infinite groups can be extremely wild but this doesn't stop us wanting to understand their general behaviour. For example, group rings were conjectured since 1940 to have very simple behaviour regarding elements like zero divisors or units. I will explain why this is not the case, as even very tame groups can support exotic units, and touch on connections to conjectures in analysis, topology and dynamics.
18:00, Discussion and Dinner
Travel funding
We have some funding available to cover train travel for graduate students and early career researchers in the region. If you would like to apply for travel funding, please contact one of the local organizers before June 8, 2026.
Contact
Martina Conte (Bielefeld): mconte(at)math.uni-bielefeld.de
Sam Hughes (Bonn): hughes(at)math.uni-bonn.de
Georges Neaime (Bielefeld): gneaime(at)math.uni-bielefeld.de
Lawk Mineh (Bonn): lawk(at)math.uni-bonn.de
Sam Shepherd (Münster): sam.shepherd(at)uni-muenster.de
Robin J. Sroka (Münster): robinjsroka(at)uni-muenster.de