1. School of Science, Department of Mathematics, Tokai University, Hiratsuka, Kanagawa 259–1292, Japan.
2. Faculty of Economics, Hosei University, 4342 Aihara-machi, Machida, Tokyo 194–0298, Japan, and Molecular and Integrative Biosciences Research Programme, Faculty of Biological and Environmental Sciences, University of Helsinki, Finland.
3. School of Science, Department of Mathematics, Tokai University, Hiratsuka, Kanagawa 259–1292, Japan. ym24896@tsc.u-tokai.ac.jp