Bounded cocycles: von Neumann algebras and amenability.

Abstract

In a 1993 preprint Guyan Robertson proved that every uniformly bounded representation of a discrete group on a finite von Neumann algebra is similar to a unitary representation. We have since discovered that this result was first proved in a paper of Vasilescu and Zsido, published in 1974 (VZ). In this thesis we generalise this result for discrete groupoids, proving that every uniformly bounded cocycle into a finite von Neumann algebra is cohomologous to a unitary cocycle. The corresponding result for cocycles into finite dimensional algebras was proved in (Zim3). We also derive some equivalent definitions of amenability of group actions and provide a new proof of a result of Zimmer regarding uniformly bounded cocycles on amenable G-spaces. We develop some machinery in order to prove these results. This is the theory of ${\cal G}$-flows in which we explore the actions of groupoids on Borel fields of sets. Our development of this theory follows that of the usual theory of flows from topological dynamics.