Rapid Decay for Principal Étale Groupoids

This work concerns a generalization of property (RD) from discrete groups to twisted étale groupoids equipped with a length function. We show that, under the assumption that the étale groupoid is principal, twisted property (RD) is equivalent to polynomial growth. This generalizes a result of Chen and Wei concerning rapid decay for metric spaces with bounded geometry. Additionally, some permanence properties of groupoid (RD) are established.