1. Department of Health Sciences, Oita University of Nursing and Health Sciences 2944-9, Megusuno, Oita-shi, Oita 870-1201, Japan
2. Department of Applied Mathematics, Tokyo University of Science 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan
3. Department of Mathematical Sciences, Graduate School of Science Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai-shi, Osaka 599-8531, Japan