Dynamic Asymptotic Dimension and Controlled Operator K - Theory

In earlier work the authors introduced dynamic asymptotic dimension, a notion of dimension for topological dynamical systems that applies to many interesting examples. In this paper, we use finiteness of dynamic asymptotic dimension to get information on the K-theory of the associated crossed product C ̊algebras: specifically, we give a new proof of the Baum-Connes conjecture for such actions. The key tool is controlled K-theory, as developed by Oyono-Oyono and the third author. Our main result is not new: it follows from work of Tu on amenable groupoids. Nonetheless, the proof is very different: it amounts to a computation of the K-theory of a crossed product which is quite independent of the topological formula posited by the Baum-Connes machinery. We have tried to keep the paper as self-contained as possible: we hope the main part of the paper will be accessible to someone with the equivalent of a first course in operator K-theory. In particular, we do not assume prior knowledge of controlled K-theory, and use a new and concrete model for the Baum-Connes conjecture with coefficients that requires no bivariant K-theory to set up.