Affiliation: | 1. Fachbereich Mathematik, Universit?t Duisburg‐Essen, Campus Essen, 45117 Essen, Germany. Phone: (+49)2011832422, Fax: (+49)2011832426;2. Department of Pure and Applied Mathematics, Graduate School of Information Science and Technology, Osaka University, Toyonaka, Osaka 560‐0043, Japan. Phone: (+81)6 6850 5299, Fax: (+81)6 6850 5327 |