1. a Institut Fourier, Laboratoire de Mathématiques Pures associ´ au C.N.R.S. , B.P. 74, Saint Martin d'Hères, F-38402, France
2. b Départment math .et statist. , Université de Montréal , C.P.6128, Succ. A, Montréal, H3C 3J7, Canada
3. c Department of Mathematics , Michigan State University , East Lansing, Michigan, 48824