Affiliation: | Department of Mathematics, University of Groningen, PO Box 407, 9747 AG Groningen, The Netherlands; Mathematics Institute, University of Leiden, Snellius, Niels Bohrweg 1, 2333 CA Leiden, The Netherlands; Philips Research, High Tech Campus 36 (M/S 2), 5656 AE Eindhoven, The Netherlands and Department of Mathematics, University of Groningen, PO Box 407, 9747 AG Groningen, The Netherlands |