Unramified cohomology of degree 3 and Noether's problem

Abstract

Let $G$ be a finite group and $W$ be a faithful representation of $G$ over {\bf C}. The group $G$ acts on the field of rational functions $\mathbf{C}(W)$. The aim of this paper is to give a description of the unramified cohomology group of degree 3 of the field of invariant functions $\mathbf{C}(W{)}^G$ in terms of the cohomology of $G$ when $G$ is a group of odd order. This enables us to give an example of a group for which this field is not rational, although its unramified Brauer group is trivial.