Rigidity properties of full groups of pseudogroups over the Cantor set

Abstract

We show that the (topological) full group of a minimal pseudogroup over the Cantor set satisfies various rigidity phenomena of topological dynamical and combinatorial nature. Our main result applies to its possible homomorphisms into other groups of homeomorphisms, and implies that arbitrary homomorphisms between the full groups of a vast class of pseudogroups must extend to continuous morphisms between pseudogroups (in particular giving rise to equivariant maps at the level of spaces). As applications, we obtain explicit obstructions to the existence of embeddings between full groups in terms of invariants of the underlying pseudogroups (the geometry of their orbital graphs, the complexity function, dynamical homology), and provide a complete descriptions of all homomorphisms within various families of groups including the Higman-Thompson groups (more generally full groups of one sided shifts of finite type), full groups of minimal $\mathbb{Z}$-actions on the Cantor set, and a class of groups of interval exchanges. We next consider a combinatorial rigidity property of groups, which formalises the inability of a group to act on any set with Schreier graphs growing uniformly subexponentially, or more generally not faster than a given function $f(n)$. For the exponential function this is a well-known consequence of property $(T)$. We use full groups to provide a source of examples of groups which satisfy this property but satisfy a strong negation of property $(T)$. A key tool used in the proofs is the study of the dynamics of the conjugation action of groups on their space of subgroups, endowed with the Chabauty topology. In particular we classify the confined and the uniformly recurrent subgroups of full groups.