Institution: | (1) Department of Mathematics, Faculty of Science, Niigata University, Ikarashi 2-8050, Niigata 950-2181, Japan;(2) Haus-Endt-Strasse 88, D-40593 Düsseldorf, Germany;(3) Department of Computer Science, University of Debrecen, P.O. Box 12, H-4010 Debrecen, Hungary |