Affiliation: | (1) Department of Mathematics, Faculty of Sciences, Tokyo Institute of Technology, 2-12-1 Oh-okayama, Meguro, Tokyo, Japan, 152-855 , JP;(2) Department of Mathematics, Faculty of Sciences, Saitama University, 255 Shimo-ookubo, Urawa, Saitama, Japan, 338-8570 (e-mail: tsatoh@rimath.saitama-u.ac.jp) , JP |