Expansivity of algebraic semigroup actions

Abstract

For a semigroup $S$ and a right $\mathbb{Z}[S]$-submodule $J\le \mathbb{Z}[S{]}^n$, we study expansivity of the algebraic action of $S$ induced on the Pontryagin dual of $\mathbb{Z}[S{]}^n/J$. We completely determine the class of semigroups for which expansivity of this action is characterized by the triviality of $J^{\perp }$, in terms of the existence of a finite subset $K\subseteq S$ such that $S=KS$. This condition is satisfied in particular by every monoid, and more generally by every semigroup with a left unital convolution Banach algebra, in which case we are able to extend the characterization of expansivity in terms of an invertibility condition when $J$ is finitely generated. We also exhibit examples of non-unital semigroups satisfying the hypotheses of our results.

20 pages, comments welcome