1. Department of Health Sciences, Oita University of Nursing and Health Sciences 2944-9, Megusuno, Oita-shi, Oita 870-1201, Japan
2. Graduate School of Economics, The University of Tokyo Research Fellow of the Japan Society for the Promotion of Science 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
3. Department of Mathematics, KTH Royal Institute of Technology SE-100 44 Stockholm, Sweden
4. Department of Mathematical Information Science, Tokyo University of Science 1-3, Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan