1. Department of Mathematics, School of Arts and Sciences, Tokyo Woman’s Christian University, 2–6–1 Zempukuji, Suginami-ku, Tokyo 167–8585, Japan.
2. Department of Materials Science and Engineering, College of Engineering, Shibaura Institute of Technology, 307 Fukasaku, Minuma-ku, Saitama-shi, Saitama 337–8570, Japan.