[1] Daido Univ, Dept Math, Nagoya, Aichi 4578530, Japan
[2] Hiroshima Inst Technol, Dept Mech Syst Engn, Saeki Ku, Hiroshima 7315193, Japan
[3] Oita Univ, Fac Educ & Welf Sci, Dannoharu Oita City 8701192, Japan
[4] Hiroshima Univ, Grad Sch Educ, Dept Math, Higashihiroshima 7398524, Japan