Invariant means for the wobbling group

Abstract

Given a metric space $(X,d)$, the wobbling group of $X$ is the group of bijections $g:X\rightarrow X$ satisfying $\sup\limits_{x\in X} d(g(x),x)<\infty$. We study algebraic and analytic properties of $W(X)$ in relation with the metric space structure of $X$, such as amenability of the action of the lamplighter group $\bigoplus_{X} \mathbf Z/2\mathbf Z \rtimes W(X)$ on $\bigoplus_{X} \mathbf Z/2\mathbf Z$ and property (T).Comment: 8 pages. v3: final version, with new presentation; to appear in the Bulletin of the BM