[1] Capital Normal Univ, Acad Multidisciplinary Studies, Beijing 100048, Peoples R China
[2] Shanghai Univ, Dept Math, Shanghai 200444, Peoples R China
[3] Brown Univ, Div Appl Math, Providence, RI 02912 USA
[4] Chinese Acad Sci, Inst Math, Acad Math & Syst Sci, Beijing 100190, Peoples R China
[5] Penn State Univ, Dept Math, University Pk, PA 16802 USA