Affiliation: | (1) Present address: Faculty of Mathematics, Bielefeld University, SFB 343, P.O. Box 8640, DW-4800 Bielefeld;(2) Present address: Faculty of Mathematics, TU Berlin, Straße des 17. Juni 135, DW-3000 Berlin 12;(3) Mathematical Institute, 24-29 St. Giles', OX1 3LB GB-Oxford, England;(4) Science Institute, University of Iceland, Dunhaga 3, IS-107 Reykjavík |