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Colin Jahel

Publications and source records attributed to Colin Jahel.

13 recordsLinked to original sources
arXiv2025

On dissociated infinite permutation groups

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), the 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 (notably some results of Tsankov). 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 countable metrically homogeneous spaces, automorphism groups of diversities, and more.

math.GR↗
arXiv2024

A non-de Finetti theorem for countable Euclidean spaces

The classical de Finetti Theorem classifies the $\mathrm{Sym}(\mathbb N)$-invariant probability measures on $[0,1]^{\mathbb N}$. More precisely it states that those invariant measures are combinations of measures of the form $\nu^{\otimes\mathbb N}$ where $\nu$ is a measure on $[0,1]$. Recently, Jahel--Tsankov generalized this theorem showing that under conditions on $M$, the group $\operatorname{Aut}(M)$ is de Finetti, i.e. $\operatorname{Aut}(M)$-invariant measures on $[0,1]^M$ are mixtures of measures of the form $\nu^{\otimes M}$ where $\nu$ is a measure on $[0,1]$. In this note, we give an example of a non-de Finetti non-Archimedean group.

math.PR↗
arXiv2024

Unitary Representations of the Isometry Groups of Urysohn Spaces

We obtain a complete classification of the continuous unitary representations of the isometry group of the rational Urysohn space $\mathbb{Q}\mathbb{U}$. As a consequence, we show that Isom$(\mathbb{Q}\mathbb{U})$ has property (T). We also derive several ergodic theoretic consequences from this classification: $(i)$ every probability measure-preserving action of Isom$(\mathbb{Q}\mathbb{U})$ is either essentially free or essentially transitive, $(ii)$ every ergodic Isom$(\mathbb{Q}\mathbb{U})$-invariant probability measure on $[0,1]^{\mathbb{Q}\mathbb{U}}$ is a product measure. We obtain the same results for isometry groups of variations of $\mathbb{Q}\mathbb{U}$, such as the rational Urysohn sphere $\mathbb{Q}\mathbb{U}_1$, the integral Urysohn space $\mathbb{Z}\mathbb{U}$, etc.

math.GR↗
arXiv2024

When invariance implies exchangeability (and applications to invariant Keisler measures)

We study the problem of when, given a countable homogeneous structure $M$ and a space $S$ of expansions of $M$, every $\mathrm{Aut}(M)$-invariant probability measure on $S$ is exchangeable (i.e. invariant under all permutations of the domain). We show, for example, that if $M$ is a finitely bounded homogeneous $3$-hypergraph with free amalgamation (including the generic tetrahedron-free $3$-hypergraph), all $\mathrm{Aut}(M)$-invariant random expansions by graphs are exchangeable. Moreover, we extend and recover both the work of Angel, Kechris, and Lyons on invariant random orderings and some of the work of Crane and Towsner, and Ackerman on relative exchangeability. In the second part of the paper, we apply our results to the study of invariant Keisler measures, which we prove to be particular invariant random expansions. Thus, we describe the spaces of invariant Keisler measures of various homogeneous structures, obtaining the first results of this kind since the work of Albert and Ensley. We also show there are $2^{\aleph_0}$ supersimple homogeneous ternary structures for which there are non-forking formulas which are universally measure zero.

math.LO↗
arXiv2024

Quasi-invariant measures concentrating on countable structures

Countable $\mathcal{L}$-structures $\mathcal{N}$ whose isomorphism class supports a permutation invariant probability measure in the logic action have been characterized by Ackerman-Freer-Patel to be precisely those $\mathcal{N}$ which have no algebraicity. Here we characterize those countable $\mathcal{L}$-structure $\mathcal{N}$ whose isomorphism class supports a quasi-invariant probability measure. These turn out to be precisely those $\mathcal{N}$ which are not "highly algebraic" -- we say that $\mathcal{N}$ is highly algebraic if outside of every finite $F$ there is some $b$ and a tuple $\bar{a}$ disjoint from $b$ so that $b$ has a finite orbit under the pointwise stabilizer of $\bar{a}$ in $\mathrm{Aut}(\mathcal{N})$. As a bi-product of our proof we show that whenever the isomorphism class of $\mathcal{N}$ admits a quasi-invariant measure, then it admits one with continuous Radon--Nikodym cocycles.

math.LO↗
arXiv2023

Stabilizers for ergodic actions and invariant random expansions of non-archimedean Polish groups

Let $G$ be a closed permutation group on a countably infinite set $\Omega$, which acts transitively but not highly transitively. If $G$ is oligomorphic, has no algebraicity and weakly eliminates imaginaries, we prove that any probability measure preserving ergodic action $G\curvearrowright (X,\mu)$ is either essentially free or essentially transitive. As this stabilizers rigidity result concerns a class of non locally compact Polish groups, our methods of proof drastically differ from that of similar results in the realm of locally compact groups. We bring the notion of dissociation from exchangeability theory in the context of stabilizers rigidity by proving that if $G\lneq\mathrm{Sym}(\Omega)$ is a transitive, proper, closed subgroup, which has no algebraicity and weakly eliminates imaginaries, then any dissociated probability measure preserving action of $G$ is either essentially free or essentially transitive. A key notion that we develop in our approach is that of invariant random expansions, which are $G$-invariant probability measures on the space of expansions of the canonical (model theoretic) structure associated with $G$. We also initiate the study of invariant random subgroups for Polish groups and prove that - although the result for p.m.p. ergodic actions fails for the group $\mathrm{Sym}(\Omega)$ of all permutations of $\Omega$ - any ergodic invariant random subgroup of $\mathrm{Sym}(\Omega)$ is essentially transitive.

math.DS↗
arXiv2022

Asymptotic Theories of Classes Defined by Forbidden Homomorphisms

We study the first-order almost-sure theories for classes of finite structures that are specified by homomorphically forbidding a set $\mathcal{F}$ of finite structures. If $\mathcal{F}$ consists of undirected graphs, a full description of these theories can be derived from the Kolaitis-Pr\"omel-Rothschild theorem, which treats the special case where $\mathcal{F} = \{K_n\}$. The corresponding question for finite sets $\mathcal{F}$ of finite directed graphs is wide open. We present a full description of the almost-sure theories of classes described by homomorphically forbidding finite sets $\mathcal{F}$ of oriented trees; all of them are $\omega$-categorical. In our proof, we establish a result of independent interest, namely that every constraint satisfaction problem for a finite digraph has first-order convergence, and that the corresponding asymptotic theory can be described as a finite linear combination of $\omega$-categorical theories.

math.CO↗
arXiv2022

Topological dynamics of Polish group extensions

We consider a short exact sequence $1\to H\to G\to K\to 1$ of Polish groups and consider what can be deduced about the dynamics of $G$ given information about the dynamics of $H$ and $K$. We prove that if the respective universal minimal flows $M(H)$ and $M(K)$ are metrizable, then so is $M(G)$. Furthermore, we show that if $M(H)$ and $M(K)$ are metrizable and both $H$ and $K$ are uniquely ergodic, then so is $G$. We then discuss several examples of these phenomena

math.DS↗
arXiv2021

Invariant measures on products and on the space of linear orders

Let $M$ be an $\aleph_0$-categorical structure and assume that $M$ has no algebraicity and has weak elimination of imaginaries. Generalizing classical theorems of de Finetti and Ryll-Nardzewski, we show that any ergodic, $\operatorname{Aut}(M)$-invariant measure on $[0, 1]^M$ is a product measure. We also investigate the action of $\operatorname{Aut}(M)$ on the compact space $\mathrm{LO}(M)$ of linear orders on $M$. If we assume moreover that the action $\operatorname{Aut}(M) \curvearrowright M$ is transitive, we prove that the action $\operatorname{Aut}(M) \curvearrowright \mathrm{LO}(M)$ either has a fixed point or is uniquely ergodic.

math.DS↗
arXiv2020

Minimal model-universal flows for locally compact Polish groups

Let $G$ be a locally compact Polish group. A metrizable $G$-flow $Y$ is called model-universal if by considering the various invariant probability measures on $Y$, we can recover every free action of $G$ on a standard Lebesgue space up to isomorphism. Weiss has shown that for countable $G$, there exists a minimal, model-universal flow. In this paper, we extend this result to all locally compact Polish groups.

math.DS↗
arXiv2019

Extending partial automorphisms of $n$-partite tournaments

We prove that for every $n\geq 2$ the class of all finite $n$-partite tournaments (orientations of complete $n$-partite graphs) has the extension property for partial automorphisms, that is, for every finite $n$-partite tournament $G$ there is a finite $n$-partite tournament $H$ such that every isomorphism of induced subgraphs of $G$ extends to an automorphism of $H$. Our constructions are purely combinatorial (whereas many earlier EPPA results use deep results from group theory) and extend to other classes such as the class of all finite semi-generic tournaments.

math.CO↗