Bordisms of manifolds with proper action of a discrete group: signatures   and descriptions of $G$-bundles
  In this work the equivariant signature of a manifold with proper action of a discrete group is defined as an invariant of equivariant bordisms. It is shown that the computation of this signature can be reduced to its computation on fixed points sets equipped with their tubular neighborhoods. It is given a description of the equivariant vector bundles with action of a discrete group $G$ for the case when the action over the base is proper quasi-free, i.e. the stationary subgroup of any point is finite. The description is given in terms of some classifying space. 
A.S. Mishchenko, Quitzeh Morales Mel\'endez
math.AT math.KT