Séminaire "Groupes et actions"

Programme automne 2025

• 15 septembre: Sasha Bontemps (ENS Lyon), "Subgroup mixing in Baumslag-Solitar groups"


• Résumé : Endowed with the Chabauty topology, the space of subgroups of any infinite countable group G is a closed subspace of the Cantor set, equipped with an action by homeomorphisms given by the G-conjugation. We are interested in the dynamics induced by this action on closed G-invariant subspaces. The largest closed subspace without isolated point is an example of such subspace called the perfect kernel of G. In an acylindrically hyperbolic context, Hull, Mynasyan and Osin demonstrated strong mixing properties (namely µ-mixing for a suitable measure µ on G, a strengthening of high topological transitivity). We uncover a radically different situation in the case of non metabelian Baumslag-Solitar groups. For the decomposition of the perfect kernel introduced by Carderi, Gaboriau, Le Maître and Stalder, who proved high topological transitivity on each piece, we show that the conjugation is even µ-mixing in the case of unimodular Baumslag-Solitar groups. On the contrary, when the group is non unimodular, there exists a continuum of measures µ for which the action is µ-mixing only on a single piece of the partition.


• 22 septembre: Pénélope Azuelos (Bristol), " Group structures on real trees and their products "


• Résumé : Median spaces form a broad and increasingly important class of metric spaces, encompassing both CAT(0) cube complexes and real trees. The study of finitely generated groups admitting free transitive (or proper cocompact) actions on discrete median spaces—equivalently, on the 0-skeletons of CAT(0) cube complexes—are an active area of study. In contrast, much less is understood about their continuous analogue: groups acting freely and transitively on connected median spaces. I will present several methods for constructing such actions, focusing on actions on real trees and their products, and discuss some of the surprising behaviours that show up. Even when considering real trees, the class of groups acting on such spaces is vastly more diverse than in the discrete setting: while any simplicial tree admits at most one free vertex transitive action, we will see that there are 2^{2^{\aleph_0}} pairwise non-isomorphic groups which admit a free transitive action on the universal real tree with continuous valence.


• 29 septembre: Federico Viola (EPFL), "Irreducible finite-index representations of automorphism groups of trees"


• Résumé : We study tree automorphism groups and their continuous irreducible representations that preserve a nondegenerate bilinear form of finite index on a Hilbert space. We show that, for groups acting transitively on the boundary of the tree and satisfying Tits' independence property (T), such representations only exist if the bilinear form has index 1.


• 6 octobre: Relâche (Séminaire à l'IHES)
• 13 octobre: Hanna Oppelmayer (Innsbruck), "Invariant random sub-von Neumann algebras"


• Résumé : The notion of IRS (invariant random subgroup) is well-studied in dynamics on groups. We extend this notion to group von Neumann algebras LG, where G is a discrete countable group. We call this concept IRA (invariant random sub-algebra). In particular, we study the case of amenable IRAs, i.e. almost every sub-von Neumann algebra of LG is amenable. This generalises a result of Bader-Duchesne-Lécureux about amenable IRSs. This is joint work with Tattwamasi Amrutam and Yair Hartman. No prior knowledge about von Neumann algebras is assumed.


• 20 octobre: Nolwenn Le Quellec (Créteil), "Parabolic and first kind flute surfaces"


• Résumé : For a surface, to be parabolic is equivalent to many things such as, the geodesic flow being ergodic, the quotient group of the surface being of divergence-type, the brownian motion on the surface being recurrent etc… In this talk we are going to describe ways to tell if a flute surface is parabolic and/or of first kind. This extends results by Pandazis and Šarić shown in 2023 and use the concept of visible end coined by Basmajian and Šarić.


• 27 octobre: Vacances
• 3 novembre: Vincent Dumoncel (IMJ-PRG), "Isoperimetric profiles, quasi-isometries and quantitative orbit equivalence between lampshufflers"


• Résumé : Lampshufflers are semi-direct products having a geometry close to the one of lamplighters. in geometric group theory, they are also a source of examples of groups with unexpected and exotic behaviours. I will present an ongoing joint work with Corentin Correia, in which we study these groups from an analytical, geometric and measured point of view. In particular, we show a stability property for the existence of orbit equivalence couplings between lampshufflers, and by computing precisely their isoperimetric profiles, extending previous results from Erschler-Zheng and Saloff-Coste-Zheng, we show that our couplings are quantitatively optimal. The computation of the isoperimetric profile also finds applications to the existence problem of quasi-isometries or regular embeddings between lampshufflers, and allows to enrich the classification program initiated recently by Genevois and Tessera.


• 10 novembre: Relâche (Séminaire à l'IHES)
• 17 novembre: Cyril Houdayer (ENS), "Weyl groups and rigidity of von Neumann algebras"


• Résumé : I will discuss a recent joint work with Adrian Ioana (UCSD) in which we recover the Weyl group of a noncompact semisimple algebraic group from natural inclusions of von Neumann algebras arising from algebraic homogeneous actions of irreducible lattices. Our main theorem is a noncommutative analogue of a rigidity result of Bader-Furman-Gorodnik-Weiss (2012) for group actions on algebraic homogeneous spaces and moreover gives new insight towards Connes' rigidity conjecture for higher rank lattices.


• 24 novembre: Milan Donvil (ENS), "W*-superrigidity for groups with infinite centre"


• Résumé : To any countable group G, one associates its group von Neumann algebra L(G), which is the closure of the group algebra in a weak topology. A central question is: "How does L(G) depend on G?" In the extreme case where L(G) completely remembers G, one says that G is W*-superrigid. Several classes of W*-superrigid groups are known, but they are all icc (infinite conjugacy classes). In particular, they are far from having centre. In fact, commutativity often poses an obstruction for rigidity. Nevertheless, in a recent work with Stefaan Vaes we were able to find the first W*-superrigid groups which have infinite centre. I will explain this result and how it relates to a previous work on W*-superrigidity for cocycle twisted group von Neumann algebras.


• 1er décembre: Alonso Beaumont (Rennes), "A uniform Tits alternative for endomorphisms of the projective line"


• Résumé : A finitely generated group G is said to have a uniform radius of independence R if, for any finite generating set S of G, there exists two elements of S-word length at most R that generate a free nonabelian subgroup; this is a measure of the ubiquity of free subgroups inside G. A group is said to satisfy the uniform Tits alternative if its finitely generated subgroups are either virtually solvable, or have a uniform radius of independence. This property was established for linear groups by E. Breuillard and T. Gelander, extending the work of J. Tits. I will explain how an analogous property can be stated for semigroups (where "free subgroup" is replaced with "free subsemigroup", and "virtually solvable" is replaced with an appropriate notion of "virtually nilpotent"), and sketch a proof of it for the semigroup of endomorphisms of the projective line. In particular, I will describe a ping-pong argument where arithmetic height functions make a remarkable appearance.


• 8 décembre: Relâche (Séminaire à l'IHES)
• Mardi 9 décembre, 11h, Salle 3L8: Yassine Guerch (Caen), "Invariants dynamiques des groupes libres-par-cycliques"


• Résumé : Un groupe G est libre-par-cyclique s'il contient un sous-groupe distingué F isomorphe à un groupe libre non abélien tel que G/F est isomorphe à un groupe cyclique infini Z. De tels groupes sont isomorphes au produit semi-direct de F par Z induit par un automorphisme de F. L'automorphisme induisant le produit semi-direct n'est cependant pas unique et nous cherchons à comprendre des propriétés dynamiques communes à tous ces automorphismes. Nous étudions en particulier la croissance de l'automorphisme, c'est-à-dire le comportement asymptotique des suites d'itérés d'éléments de F. Nous montrons ainsi que les laminations attractives d'un automorphisme, qui sont des objets dynamiques associés à la croissance exponentielle de l'automorphisme, induisent des invariants algébriques du groupe ambiant G. Travail en commun avec S. Dowdall, R. Gupta, J.P. Mutanguha et C. Uyanik.


• 15 décembre: Corentin Le Bars (ENS), "Groups acting on buildings and their C*-algebras"


• Résumé : Let G be a group acting on either a symmetric space of noncompact type or an affine building X. I will describe some properties of the induced dynamics on the spherical building at infinity X^\infty. In many situations, G admits elements that satisfy a weak form of North-South dynamics. These elements, along with the construction of a barycenter map on triples of chambers at infinity, can help understand the C*-algebra of these groups. In particular, I will show that, even if the model space is an exotic affine building, lattices are C*-simple, and that lattices in model spaces of type A_2 (whose buildings at infinity are projective planes) are selfless. In particular, SL_3(Z) is selfless, providing the first example of a non-uniform lattice in higher rank satisfying this property. Joint work with C. Ciobotaru, I. Vigdorovich and E. Leibtag. 


• 5 janvier: Marie Trin (Liepzig), "Stable subgroups of graph products"


• Résumé : Stable subgroups of non-hyperbolic groups have been introduced by Durham and Taylor as a well-defined notion of convexity in non-hyperbolic groups. For different examples of groups, stable subgroups are characterized through a well-known action on a hyperbolic space. We will describe such characterizations of stable subgroups in RAAGs and mapping class groups and will see what are the analogs for graph products. This is based on joint work with S. H. Balasubramanya, M. Chesser, A. Kerr and J. Mangahas.


• 12 janvier: Relâche (Séminaire à l'IHES)


• 19 janvier: Joaquin Brum (Montevideo), TBA


• 26 janvier: TBA


• 2 février: Siwei Liang (Orsay), TBA


• 9 février: TBA


• 16 février: Alex Loué (Louvain-la-Neuve), TBA


• 23 février: TBA


• 2 mars: Vacances