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.
4th Meeting - Münster, 30th April 2026
Local organizers: Sam Shepherd and Robin J. Sroka.
Location and directions:
This meeting takes place at the Conference Centre of Cluster of Excellence "Mathematics Münster" at the University of Münster. All talks are scheduled in Seminar Room SRZ 216/217 on the second floor of the Seminarraumzentrum (SRZ), Orléans-Ring 12, 48149 Münster. For precise location and travel directions, please click HERE.
Registration:
No registration is needed to participate in this event.
Schedule:
13:00, Arrival
13:30-14:30, Thor Wittich (University of Osnabrück)
The Spherical Hopf Algebra
In spherical geometry, classical work of Sah shows that polytopes can be assembled into a commutative Hopf algebra, now called Sah algebra. In joint work with Klang, Kuijper, Malkiewich and Mehrle, we study this Hopf algebra from a homotopical point of view and show that the Hopf algebra structure also exists on the level of cohomology theories. We will hide most of the homotopy theory by rephrasing pretty much everything in terms of group cohomology, so do not worry about your homotopical background!
14:30-15:00, Conference Picture & Coffee Break
15:00-16:00, Stefanie Zbinden (University of Bonn)
Relating different notions of non-positive curvature
The study of non-positive curvature in groups is a major theme in geometric group theory, and there are now many competing definitions. In this talk, we take the perspective of studying the large-scale behaviours of geodesics, and compare the primary classes of groups that arise in this study, namely acylindrically hyperbolic and Morse local-to-global groups. We then show that Morse local-to-global groups are characterised by a compactness condition on its Morse boundary.
16:00-16:30, Coffee Break
16:30-17:30, Elia Fioravanti (Karlsruhe Institute of Technology)
Generators for automorphisms of special groups
Given a family F of finitely generated groups, do all groups in F have "tame" automorphisms, or can there be "wild" examples? More concretely, is Out(G) finitely generated for all groups G in the family F? Rips and Sela showed in the 90s that Out(G) is finitely generated for all Gromov-hyperbolic groups G, while Baues and Grunewald showed in the 00s that Out(G) is arithmetic over Q (and hence finitely generated) for all virtually polycyclic groups G. This essentially exhausts our limited understanding of general phenomena of this kind, with the structure of automorphisms of non-positively curved groups remaining a fundamental open problem. I will discuss the recent result that Out(G) is finitely generated for all (cocompact) special groups of Haglund and Wise. This is already new for most finite-index subgroups of right-angled Artin and Coxeter groups.
17:30, Discussion and Dinner
Travel funding
We have some funding available to cover train travel for graduate students and early career researchers in the region. Please contact one of the local organizers if you would like to apply for travel funding.
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