Periodic cyclic cohomology of group rings

We generalize previous results ([2], [3], [4], etc.) relative to the cyclic homology and cohomology of the group algebra of G. In many cases, we express them in terms of the (co)homology of the discrete groups(u) = Z (u)/C (u), where 〈u〉 runs through the set of conjugacy classes of G and where Z(u) (resp. C(u)) denotes the centralizer of u (resp. the cyclic group generated by u).