Affiliation: | (1) Department of Mathematics, School of Science and Engineering, Waseda University, 3-4-1, Ohkubo Shinjuku-ku, Tokyo 169, Japan , JP;(2) Department of Applied Mathematics, Faculty of Science, Fukuoka University, Nanakuma 8-19-1, Jonan-ku, Fukuoka 814-01, Japan , JP;(3) Department of Mathematics, Keio University, 3-14-1, Hiyoshi, Yokohama 223, Japan (e-mail: tani@math.keio.ac.jp) , JP |