Quand l'homologie cyclique périodique n'est pas la limite projective de l'homologie cyclique. (When periodic cyclic homology is not the projective limit of cyclic homology) (Q1118684)

After recalling results and general points on cyclic homology of group rings, this paper considers the cyclic group \({\mathbb{Z}}/_ 2\) and the group rings \({\mathbb{Z}}[{\mathbb{Z}}/_ 2]\) and \({\mathbb{Z}}[{\mathbb{Z}}/_ n]\) and calculates their periodic cyclic cohomologies. The author shows here that periodic cyclic homology is generally not the inverse limit of cyclic homology and not, unlike cyclic homology, invariant under flat base change.