Affiliation: | (1) Institute of Mathematics, University of Tsukuba, Tsukuba-shi, Ibaraki 305, Japan (e-mail: kobayashi@math.tsukuba.ac.jp) , JP;(2) Department of Mathematics, School of Science and Engineering, Waseda University, 3-4-1 Ohkubo, Shinjyuku, Tokyo 169, Japan (e-mail: yshibata@mn.waseda.ac.jp) , JP |