Locally approximating groups of homeomorphisms of manifolds

Abstract

Let $M$ be a compact, connected manifold of positive dimension and let $\mathcal G\leq\textrm{Homeo}(M)$ be \emph{locally approximating} in the sense that for all open $U\subseteq M$ compactly contained in a single Euclidean chart of $M$, the subgroup $\mathcal G[U]$ consisting of elements of $\mathcal G$ supported in $U$ is dense in the full group of homeomorphisms supported in $U$. We prove that $\mathcal G$ interprets first order arithmetic, as well as a first order predicate that encodes membership in finitely generated subgroups of $\mathcal G$. As a consequence, we show that if $\mathcal G$ is not finitely generated, then no group elementarily equivalent to $\mathcal G$ can be finitely generated. We show that many finitely generated locally approximating groups of homeomorphisms $\mathcal G$ of a manifold are prime models of their theories, and give conditions that guarantee any finitely presented group $G$ that is elementarily equivalent to $\mathcal G$ is isomorphic to $\mathcal G$. We thus recover some results of Lasserre about the model theory of Thompson's groups $F$ and $T$. Finally, we obtain several action rigidity result for locally approximating groups of homeomorphisms. If $\mathcal G$ acts in a locally approximating way on a compact, connected manifold $M$ then the dimension of $M$ is uniquely determined by the elementary equivalence class of $\mathcal G$. Moreover, if $\dim M\leq 3$ then $M$ is uniquely determined up to homeomorphism. In for general closed smooth manifolds, the homotopy type of $M$ is uniquely determined. In this way, we obtain a generalization of a well-known result of Rubin.

Theorems & Definitions (62)

• Theorem 1.1
• Theorem 1.2
• Theorem 1.3
• Corollary 1.4
• Theorem 1.5
• Corollary 1.6
• Corollary 1.7
• Theorem 1.8
• Corollary 1.9
• Theorem 1.10