Lamplighters and the bounded cohomology of Thompson’s group

Abstract

We prove the vanishing of the bounded cohomology of lamplighter groups for a wide range of coefficients. This implies the same vanishing for a number of groups with self-similarity properties, such as Thompson’s group F. In particular, these groups are boundedly acyclic. Our method is ergodic and applies to “large” transformation groups where the Mather–Matsumoto–Morita method sometimes fails because not all are acyclic in the usual sense.