Geometric Property (T) and Kazhdan projections

Abstract

We characterise Geometric Property (T) by the existence of a certain projection in the maximal uniform Roe algebra $C_{u,\max }^{\ast }(X)$, extending the notion of Kazhdan projection for groups to the realm of metric spaces. We also describe this projection in terms of the decomposition of the metric space into coarsely connected components.

Final version. 13 pages. Several improvements in the presentation, following the referee's comments. Results unchanged. Accepted for publication in Bulletin of the London Mathematical Society