The Furstenberg Boundary of a Groupoid

Abstract

We define the Furstenberg boundary of a locally compact Hausdorff \'etale groupoid, generalising the Furstenberg boundary for discrete groups, by providing a construction of a groupoid-equivariant injective envelope. Using this injective envelope, we establish the absence of recurrent amenable subgroups in the isotropy as a sufficient criterion for the intersection property of a locally compact Hausdorff \'etale groupoid with compact unit space and no fixed points. This yields a criterion for C*-simplicity of minimal groupoids.