Exact groupoids

Abstract: Our purpose is to introduce in the setting of locally compact groupoids the analogues of the well known equivalent definitions of exactness for discrete groups. We study the relations between these notions. The best results are obtained for a class of \'etale groupoids that we call inner amenable, since for locally compact groups this notion coincides with the notion of inner amenability. We give examples of such groupoids which include transformation groupoids associated to actions of discrete groups by homeomorphisms on locally compact spaces. We have no example of \'etale groupoids which are not inner amenable. For inner amenable \'etale groupoids we extend what is known for discrete groups in proving the equivalence of six natural notions of exactness: (1) strong amenability at infinity; (2) amenability at infinity; (3) nuclearity of the uniform (Roe) algebra of the groupoid; (4) exactness of this $C^*$-algebra; (5) exactness of the reduced $C^*$-algebra; (6) exactness of the groupoid in the sense of Kirchberg-Wassermann. We give several illustrations of our results and review the results obtained by several authors that highlight the crucial role of exactness in order to clarify the relation between the amenability of a groupoid and the fact that its full and reduced $C^*$-algebras coincide. We end our paper with open questions and an appendix on fibrewise compactifications because our work requires to extend from discrete groups to \'etale groupoids the notion of Stone-Cech compactification on which the groupoid acts.