(1) Present address: Fachbereich Mathematik, Universität Mainz, Saarstraße 21, D-6500 Mainz;(2) Present address: Dipartimento di Mathematica, Università di Napoli, Via Mezzocannone 8, 1-80134 Napoli, Italy
Abstract:
This research was done while the last two authors were visitors at the University of Mainz. They are grateful to the Department of Mathematics for its excellent hospitality.