New algebras of functions on topological groups arising from G-spaces
Eli Glasner, Michael Megrelishvili
TL;DR
The paper introduces SUC(G), the algebra of strongly uniformly continuous functions, and situates it within the usual hierarchy $UC(G) \supset SUC(G) \supset LE(G) \supset Asp(G) \supset WAP(G)$. It provides matrix-coefficient characterizations via Banach space representations, and builds a framework of cyclic G-systems to analyze G-compactifications, highlighting the universal SUC compactification. The authors compute explicit ROE-type results for key groups: for $S({\mathbb N})$ one has $SUC(G)=UC(G)=WAP(G)$ with a Cantor-type metrizable semitopological semigroup compactification, while for $H(C)$ one has $SUC(G)\subsetneq UC(G)$ and the Roelcke compactification fails to be a right-topological semigroup compactification. They further establish strong rigidity phenomena: groups such as $H_+[0,1]$ and ${\mathrm{Iso}}({\mathbb U}_1)$ are SUC-trivial (and LE-trivial), while several Polish groups are SUC-extremely amenable; they develop a notion of relative extreme amenability (SUC-fpp) and relate it to metrizability of the universal minimal flow, providing a unified, representation-theoretic approach to dynamics on large non-locally-compact groups.
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.