1. Institute of Mathematics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571, Japan
2. Liberal Arts Education Center, Ashikaga University, 286-1 Omae-cho, Ashikaga-shi, Tochigi 326-8558, Japan
3. Global Education Center, Waseda University, 1-6-1 Nishi-Waseda, Shinjuku, Tokyo 169-8050, Japan