1Department of Applied Mathematics Science University of Tokyo 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162 Japan
2Department of Mathematics, Keio Univ., 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, 223 Japan
3Department of Computer Science, Meiji Univ., 1-1-1 Higashimita, Tama-ku, Kawasaki, 214 Japan