1. Department of Mathematics, University of Kaiserslautern, P. O. Box 3049, 67653 Kaiserslautern, Germany
2. Mathematics Department, Bielefeld University, P. O. Box 100131, 33501 Bielefeld, Germany
3. Department of Mathematics, The University of Reading, P. O. Box 220, Reading, Berkshire RG6 6AX, United Kingdom