1. Hokkaido University, Department of Mathematics, Kita 10 Nishi 8, Kita-ku, Sapporo 060-0810, Japan
2. Hokkaido University, Research Institute for Electronic Science, Kita 12 Nishi 6, Kita-ku, Sapporo 060-0812, Japan
3. Yamaha Motor Co., Ltd., Technical Development Division, 2500 Shingai, Iwata 438-8501, Japan