Publications
- On dissociated infinite permutation groups, with Colin Jahel and Matthieu Joseph, 2025. Preprint
[ Abstract | ArXiv | HAL]
The goal of this paper is threefold. First, we describe the notion of dissociation for closed subgroups of the group of permutations on a countably infinite set and explain its numerous consequences on unitary representations (classification of unitary representations, Property (T), Howe-Moore property, etc.) and on ergodic actions (non-existence of type III non-singular actions, Stabilizer rigidity, etc.). Some of the results presented here are new, others were proved in different contexts. Second, we introduce a new method to prove dissociation. It is based on a reinforcement of the classical notion of strong amalgamation, where we allow to amalgamate over countable sets. Third, we apply this technique of amalgamation to provide new examples of dissociated closed permutation groups, including isometry groups of some metrically homogeneous graphs, automorphism groups of diversities, and more.
- Unitary Representations of the Isometry Groups of Urysohn Spaces, with Colin Jahel and Matthieu Joseph, 2024. Submitted
[ Abstract | ArXiv | HAL]
We obtain a complete classification of the continuous unitary representations of the isometry group of the rational Urysohn space \(\mathbb{QU}\). As a consequence, we show that Isom(\(\mathbb{QU}\)) has property (T). We also derive several ergodic theoretic consequences from this classification: (i) every probability measure-preserving action of Isom(\(\mathbb{QU}\)) is either essentially free or essentially transitive, (ii) every ergodic Isom(\(\mathbb{QU}\))-invariant probability measure on \([0,1]^{\mathbb{QU}}\) is a product measure. We obtain the same results for isometry groups of variations of \(\mathbb{QU}\), such as the rational Urysohn sphere \(\mathbb{QU}_1\), the integral Urysohn space \(\mathbb{ZU}\), etc.
- Tannaka-Krein duality for Roelcke-precompact non-archimedean Polish groups, 2024. Submitted
[ Abstract | ArXiv | HAL]
Let \(G\) be a Roelcke-precompact non-archimedean Polish group, \(\mathcal{B}(G)\) the algebra of matrix coefficients of \(G\) arising from its continuous unitary representations. The Gel’fand spectrum \(H(G)\) of the norm closure of \(\mathcal{B}(G)\) is known as the Hilbert compactification of \(G\). Let \(\mathcal{A}_G\) be the dense subalgebra of \(\mathcal{B}(G)\) generated by indicator maps of open cosets in \(G\). We prove that multiplicative linear functionals on \(\mathcal{A}_G\) are automatically continuous, generalizing a result of Krein for finite dimensional representations of topological groups. We deduce two abstract realizations of \(H(G)\). One is the space \(P(\mathcal{M}_G)\) of partial isomorphisms with algebraically closed domain of \(\mathcal{M}_G\), the countable set of open cosets of \(G\) seen as a homogeneous first order logical structure. The other is \(T(G)\) the Tannaka monoid of \(G\). We also obtain that the natural functor that sends \(G\) to the category of its representations is full and faithful.