Maximal equivariant compactifications

TL;DR

This paper characterizes the maximal G-proximity and G-compactification for G-spaces, especially for isometry groups of the Urysohn sphere and automorphism groups of erlih structures, linking topological and uniform structures.

Contribution

It provides a new characterization of maximal G-proximity and G-compactification for locally compact and non-locally compact groups, extending to erlih structures.

Findings

Maximal G-proximity characterized via neighborhoods and topological proximity.

For the Urysohn sphere, G-proximity relates to positive distance between images.

Applicable to automorphism groups of erlih metric structures.

Abstract

Let $G$ be a locally compact group. Then for every $G$-space $X$ the maximal $G$-proximity $\beta_G$ can be characterized by the maximal topological proximity $\beta$ as follows: $$ A \ \overline{\beta_G} \ B \Leftrightarrow \exists V \in N_e \ \ \ VA \ \overline{\beta} \ VB. $$ Here, $\beta_G \colon X \to \beta_G X$ is the maximal $G$-compactification of $X$ (which is an embedding for locally compact $G$), $V$ is a neighborhood of $e$ and $A \ \overline{\beta_G} \ B$ means that the closures of $A$ and $B$ do not meet in $\beta_G X$. Note that the local compactness of $G$ is essential. This theorem comes as a corollary of a general result about maximal $\mathcal{U}$-uniform $G$-compactifications for a useful wide class of uniform structures $\mathcal{U}$ on $G$-spaces for not necessarily locally compact groups $G$. It helps, in particular, to derive the following result. Let $(\mathbb{U}_1,d)$ be the Urysohn sphere and $G=Iso(\mathbb{U}_1,d)$ is its isometry group with the pointwise topology. Then for every pair of subsets $A,B$ in $\mathbb{U}_1$, we have $$ A \ \overline{\beta_G} \ B \Leftrightarrow \exists V \in N_e \ \ \ d(VA,VB) > 0. $$ More generally, the same is true for any $\aleph_0$-categorical metric $G$-structure $(M,d)$, where $G:=Aut(M)$ is its automorphism group.