a1 Department of Mathematics University of Virginia Charlottesville, Va 22903, USA
1. The subject-matter of this paper is in some sense known; but we will try to organise, explain and reprove it, and to give examples.
(Received July 28 1988)
* The Society is saddened by the sudden death on 7 January 1989 of Professor J. F. Adams, F.R.S.