1. Department of Mathematics and Computer Science, Institute for Electronic Systems, Aalborg University, Fredrik Bajers Vej 7, DK-9220 Aalborg Ø, Denmark
2. Current address: Mittag-Leffler Institute, Auravägen 17, S-182 62 Djursholm, Sweden
3. Department of Mathematics, Hokkaido University, Sapporo 060, Japan