Measured and Geometric Group Theory, Rigidity, Operator Algebras

​Théorie mesurée et géométrique des groupes, rigidité, algèbres d’opérateurs

HYBRID RESEARCH SCHOOL /  5 – 9 October 2020
Organizing Committee

Damien Gaboriau (CNRS – ENS Lyon)
Cyril Houdayer (CNRS – Université Paris-Saclay)

Nóra Gabriella Szőke (Université de Grenoble Alpes)
Romain Tessera (CNRS – Sorbonne Université)

Scientific Committee

Uri Bader (Israel Institute of Technology)
Roman Sauer (Karlsruhe Institute of Technology)
Tianyi Zheng (Univerty of California San Diego)

(Marseille, France local time)

Measure versus topological dynamics have witnessed dramatic progress in the last ten years, and fascinating parallels have be drawn between these two theories. For instance the notion of full group became central in both settings. Similarly, the collection of stabilizers for non free actions led to the emerging topic of invariant random subgroups (IRS) and their topological counterparts : uniform recurrent subgroups (URS).
Geometric group theory and measured group theory consist in a kind of « sociological » study of groups via Gromov’s notions of topological couplings, resp. measure equivalence, and is intimately related with topological, resp. measured, dynamics of groups. Recently, a notion has been intensely studied that interpolates between measured and topological dynamics : Lp-measure equivalence.
Rigidity phenomena are omnipresent both in measure and geometric group theory in particular when dealing with higher rank lattices. Since the fundamental work of Margulis, an impressive amount of work has been devo- ted to generalize and strengthen it. Recently, various Von Neumann versions of Margulis’ theorem that had been suggested by Connes have been proved. In spite of these impressive progresses, many natural problems remain wide open, for instance very little is known for irreducible lattices in product of rank 1 simple Lie groups. Lastly, full groups have proved to produce rigid situations in terms of IRS and URS. Meanwhile, along the last years various phenomena of dizzying flexibility have been discovered especially among IRS.
The goal of this fall school is to immerse young researchers in these beau- tiful and quickly evolving subjects through 4 mini-courses completed by 6 additional one-hour talks. Special slots will be dedicated to exercise sessions.
Discussion rooms will be available to registered participants via the link above. Your access code to the rooms is provided by the organizers.
This page is accessible with a password issued by CIRM.


Rémi Boutonnet (Université de Bordeaux)   Character rigidity and non-commutative ergodic theory
Romain Tessera (CNRS – Sorbonne Université)  Quantitative measure equivalence
Adrien Le Boudec (ENS Lyon) / Nicolas Matte Bon (ICJ Université de Lyon)   Topological dynamics of non-free actions
François Le Maître (Université Paris Diderot)   Totipotent IRS of free groups


Mikael de la Salle (CNRS ENS Lyon)   Group actions on Lp spaces : dependence on p
Camille Horbez (CNRS Université Paris-Saclay)   Measure equivalence and right-angled Artin groups
Waltraud Lederle (Université Catholique de Louvain)   
Almost automorphisms of trees and completions of Thompson’s V  
Amine Marrakchi (ENS Lyon)   Ergodic theory of affine isometric actions on Hilbert spaces
Tianyi Zheng (University of California, San Diego) FC-central extensions as a source of examples