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.
3rd Meeting - Bonn, 31st October 2025
Local organisers: Sam Hughes and Lawk Mineh.
Registration
No registration is needed to participate in this event.
Location
The event will happen in the "Lipschitz-Saal". This is in the Mathematical Institute, Endenicher Allee 60. This is a 10-to-15 minute walk from the Hauptbahnhof, or a five minute bus ride (604, 605, 606, 607) and a one minute walk.
Schedule
13:00: Arrival
13:30: Laura Ciobanu
On equations in free-by-cyclic groups
For a group or semigroup or ring G, solving equations where the coefficients are elements in G and the solutions take values in G can be seen as akin to solving systems of linear equations in linear algebra, Diophantine equations in number theory, or more generally, polynomial systems in algebraic geometry.
In this talk I will present some work in progress (with Bob Gray and Alex Levine) on the decidability of solving equations in free-by-cyclic groups, with or without constraints.
14:30: Break
15:00: Anthony Genevois
Towards a stringy geometry
After a general introduction to coarse geometry and lamplighter graphs, I will explain how a coarsification of the topological notion of local cut point can be used in order distinguish, up to quasi-isometry, some wreath products of groups (and more generally some lamplighter graphs).
16:00: Break
16:30: Amandine Escalier
Quantitative measure equivalence and graph products
Measure equivalence was introduced by Gromov in the 90’s. Although presented as a measure theoretical analogue of quasi-isometry, measure equivalence sometimes crushes two groups with very different geometries to a same equivalence class. A quantitative version of measure equivalence was thus introduced: it aims at specifying how close the two equivalent actions are.
In this talk we will focus on the behaviour of graph products under quantitative measure equivalence. Through this study, we will highlight the links between the latter notion and geometric properties. This is joint work with Camille Horbez.
17:30: Dinner and discussion
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
Sam Hughes (Bonn): hughes(at)math.uni-bonn.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