Property RD and the classification of traces on reduced group C∗-algebras of hyperbolic groups

In this paper, we show that if [Formula: see text] is a non-elementary word hyperbolic group, [Formula: see text] is an element, and the conjugacy class of [Formula: see text] is infinite, then all traces [Formula: see text] vanish on [Formula: see text]. Moreover, we completely classify all traces by showing that traces [Formula: see text] are linear combinations of traces [Formula: see text] given by [Formula: see text] where [Formula: see text] is an element with finite conjugacy class, denoted [Formula: see text]. We demonstrate these two statements by introducing a new method to study traces that uses Sobolev norms and the rapid decay property.

The rapid decay property and centroids in groups

This is a survey of methods of proving or disproving the rapid decay property in groups. We present a centroid property of group actions on metric spaces. That property is a generalized (and corrected) version of the "(**)-relative hyperbolicity" from [9] and implies the rapid decay (RD) property. We show that several properties which are known to imply RD also imply the centroid property. Thus uniform lattices in many semi-simple Lie groups, graph products of groups, Artin groups of large type and the mapping class groups have the (relative) centroid property. We also present a simple "non-amenability-like" property that follows from RD, and give an easy example of a group without RD and without any amenable subgroup with superpolynomial growth.

RAPID DECAY IS PRESERVED BY GRAPH PRODUCTS

We prove that the rapid decay property (RD) of groups is preserved by graph products defined on finite simplicial graphs.