New algebras of functions on topological groups arising from G-spaces

TL;DR

This paper introduces the algebra SUC(G) of strongly uniformly continuous functions on topological groups, explores its properties and relations to other function algebras, and studies implications for group compactifications and amenability.

Contribution

It defines the new algebra SUC(G), compares it with existing algebras, and analyzes its role in group compactifications and amenability properties.

Findings

SUC(G) contains WAP(G), LE(G), Asp(G)

For certain groups, SUC(G) is trivial or equals UC(G)

Metrizable G are SUC-amenable, proximal G are SUC-extremely amenable

Abstract

For a topological group G we introduce the algebra SUC(G) of strongly uniformly continuous functions. It contains the algebra WAP(G) of weakly almost periodic functions as well as the algebras LE(G) and Asp(G) of locally equicontinuous and Asplund functions respectively. For the Polish groups of order preserving homeomorphisms of the unit interval and of isometries of the Urysohn space of diameter 1, SUC(G) is trivial. We study the Roelcke algebra (= UC(G) = right and left uniformly continuous functions) and SUC compactifications of the groups S(N), of permutations of a countable set, and H(C), the group of homeomorphisms of the Cantor set. For the first group we show that WAP(G)=SUC(G)=UC(G) and also provide a concrete description of the corresponding metrizable (in fact Cantor) semitopological semigroup compactification. For the second group, in contrast, we show that SUC(G) is properly contained in UC(G) and for this group UC(G) does not yield a right topological semigroup compactification. We introduce the notion of fixed point on a class P of flows (P-fpp) and study in particular groups which are SUC-amenable and groups with the SUC-fpp (SUC-extreme amenability). We show that every Polish group G with metrizable M(G) is SUC-amenable and if, in addition, M(G) is proximal, then G is SUC-extremely amenable.