Polish topometric groups

Abstract:

We define and study the notion of ample metric generics for a Polish topological group, which is a weakening of the notion of ample generics introduced by Kechris and Rosendal. Our work is based on the concept of a Polish topometric group, defined in this article. Using Kechris and Rosendal’s work as a guide, we explore the consequences of ample metric generics (or, more generally, ample generics for Polish topometric groups). Then we provide examples of Polish groups with ample metric generics, such as the isometry group $\operatorname {Iso}(\mathbf {U}_1)$ of the bounded Urysohn space, the unitary group ${\mathcal U}(\ell _2)$ of a separable Hilbert space, and the automorphism group $\operatorname {Aut}([0,1],\lambda )$ of the Lebesgue measure algebra on $[0,1]$. We deduce from this and earlier work of Kittrell and Tsankov that this last group has the automatic continuity property, i.e., any morphism from $\operatorname {Aut}([0,1],\lambda )$ into a separable topological group is continuous.