Affiliation: | (1) Department of Mathematics, Yamanashi University, Takeda, 400 Kofu, Japan;(2) Matsushita Electric Industrial CO. LTD., 1006 Oaza-Kadoma, 571 Kadoma, Osaka, Japan;(3) Department of Mathematics, Faculty of Science and Technology, Keio University, Kohoku-ku, 223 Yokohama, Kanagawa, Japan |