[1] Tokyo Univ Sci, Fac Sci & Technol, Dept Math, Noda, Chiba 2788510, Japan
[2] Yokohama Natl Univ, Dept Appl Math, Fac Engn, Yokohama, Kanagawa 2408501, Japan
[3] Hiroshima Univ, Grad Sch Engn, Dept Appl Math, Higashihiroshima 7398527, Japan
[4] Harbin Finance Univ, Fac Fdn, Dept Math, Harbin 150030, Heilongjiang, Peoples R China