Affiliation: | Graduate School of Mathematics, Kyushu University, Ropponmatsu 4-2-1, Fukuoka 810, Japan ; Department of Mathematics, Shinshu University, Asahi 3-1-1, Matsumoto 390, Japan Toshio Sumi ; Department of Art and Information Design, Kyushu Institute of Design, Shiobaru 4-9-1, Fukuoka 815, Japan |