1. Department of Mathematics, Graduate School of Science and Engineering, Chuo University, Bunkyo-ku, Tokyo 112-8551, Japan rykawasumi@gmail.com
2. Department of Mechanical Systems Engineering, Graduate School of Science and Engineering, Ibaraki University, Hitachi, Ibaraki 316-8511, Japan koujin.takeda.kt@vc.ibaraki.ac.jp