Institut für Geometrie

Technische Universität Dresden

01602 Dresden

Germany

Office B 121 (Willersbau)

+49 351 463-34267

maxime.gheysens AD tu-dresden.de

(My epfl.ch address will soon cease to be valid.)

My research interests lie in geometric group theory. More precisely, I work in amenability, metric spaces of nonpositive curvature, and bounded cohomology. I'm also interested in the structure theory of locally compact (totally disconnected) groups, property (T), and orbit equivalence.

I did my PhD under the supervision of Nicolas Monod (EGG chair, EPFL, Switzerland). I am now a postdoc in Andreas Thom's chair of geometry (TU Dresden, Germany).

- Representing groups against all odds (my thesis)

pdf |We investigate how probability tools can be useful to study representations of nonamenable groups. A suitable notion of 'probabilistic subgroup' is proposed for locally compact groups, and is valuable to induction of representations. Nonamenable groups admit nonabelian free subgroups in that measure-theoretical sense. Consequences for affine actions and for unitarizability are then drawn. In particular, we obtain a new characterization of amenability via some affine actions on Hilbert spaces.

Along the way, various fixed-point properties for groups are studied. We also give a survey of several useful facts about group representations on Banach spaces, continuity of group actions, compactness of convex hulls in locally convex spaces, and measurability pathologies in Banach spaces.

*Keywords:*amenability, group representation, fixed-point property, von Neumann problem, Dixmier problem, induced representation, tychomorphism, Krein space. - The scale function and tidy subgroups (expository paper)

with A. Brehm, A. Le Boudec, and R. Rollin

arXiv |This exposition article arose from two talks given during the Oberwolfach Arbeitsgemeinschaft on Totally Disconnected Groups in October 2014.

This is an introduction to the structure theory of totally disconnected locally compact groups initiated by Willis in 1994. The two main tools in this theory are the scale function and tidy subgroups, for which we present several properties and examples.

As an illustration of this theory, we give a proof of the fact that the set of periodic elements in a totally disconnected locally compact group is always closed, and that such a group cannot have ergodic automorphisms as soon as it is non-compact. - Fixed points for bounded orbits in Hilbert spaces

with N. Monod

arXiv |Consider the following property of a topological group G: every continuous affine G-action on a Hilbert space with a bounded orbit has a fixed point. We prove that this property characterizes amenability for locally compact σ-compact groups (e.g. countable groups).

Along the way, we introduce a "moderate" variant of the classical induction of representations and we generalize the Gaboriau–Lyons theorem to prove that any non-amenable locally compact group admits a probabilistic variant of discrete free subgroups. This leads to the "measure-theoretic solution" to the von Neumann problem for locally compact groups.

We illustrate the latter result by giving a partial answer to the Dixmier problem for locally compact groups.

Annales scientifiques de l'École normale supérieure, vol.**50**(2017), p. 131–156.

- Ergodic and Geometric Group Theory Seminar (EPFL, Switzerland), December 2017
- Séminaire de Géométrie, Groupes et Dynamiques (ENS de Lyon, France), June 2017
- Seminar Geometrie (TU Dresden, Germany), January 2017
- Séminaire de théorie des groupes (UCL, Belgium), January 2017
- Séminaire « Groupes et Géométrie » (Université de Genève, Switzerland), November 2016
- Geometry Seminar (ETHZ, Switzerland), October 2016
- Journées d'analyse fonctionnelle Besançon–Neuchâtel (Université de Franche-Comté, France), May 2016
- Séminaire de Géométrie Différentielle (Université de Lorraine, France), May 2016
- Geometry and Analysis on Groups Research Seminar (Universität Wien, Austria), January 2016
- Oberseminar C*-Algebren (WWU Münster, Germany), January 2016
- Groups and Analysis Seminar (Université de Neuchâtel, Switzerland), October 2015
- Ergodic and Geometric Group Theory Seminar (EPFL, Switzerland), May 2015

I supervised some student projects during my time at EPFL. See the related webpage for student projects in EGG chair.

- Addressing the underrepresentation of women in mathematics conferences, by Greg Martin
- Le libre accès à la recherche : une introduction, by Pablo Rauzy (in French)
- Falso, the right axiomatic system, by Estatis Inc, the commercial branch of the Club inutile

Last update: November 7, 2017.