The dynamics of partial inverse semigroup actions

Abstract

Given an inverse semigroup S endowed with a partial action on a topological space X, we construct a groupoid of germs S⋉X in a manner similar to Exel's groupoid of germs, and similarly a partial action of S on an algebra A induces a crossed product A⋊S. We then prove, in the setting of partial actions, that if X is locally compact Hausdorff and zero-dimensional, then the Steinberg algebra of the groupoid of germs S⋉X is isomorphic to the crossed product AR(X)⋊S, where AR(X) is the Steinberg algebra of X. We also prove that the converse holds, that is, that under natural hypotheses, crossed products of the form AR(X)⋊S are Steinberg algebras of appropriate groupoids of germs of the form S⋉X. We introduce a new notion of topologically principal partial actions, which correspond to topologically principal groupoids of germs, and study orbit equivalence for these actions in terms of isomorphisms of the corresponding groupoids of germs. This generalizes previous work of the second-named author as well as from others, which dealt mostly with global actions of semigroups or partial actions of groups. We finish the article by comparing our notion of orbit equivalence of actions and orbit equivalence of graphs.