Affiliation: | (1) Dept. of Mathematical and Computing Sciences, Tokyo Institute of Technology, 2-12-1 Oh-Okayama, Meguro-ku, Tokyo 152, Japan, e-mail: kojima@is.titech.ac.jp, JP;(2) Dept. of Mathematics, Faculty of Engineering, Kanagawa University, Rokkakubashi, Kanagawa-ku, Yokohama 221, Japan, e-mail: shida@is.titech.ac.jp, JP;(3) Dept. of Mathematics and Physics, The National Defense Academy, Hashirimizu 1-10-20, Yokosuka 239, Japan, e-mail: shindoh@cc.nda.ac.jp, JP |