[1] Chinese Acad Sci, Acad Math & Syst Sci, HCMS, HLM, Beijing 100190, Peoples R China
[2] Univ Chinese Acad Sci, Sch Math Sci, Beijing 100049, Peoples R China
[3] Beijing Normal Univ, MOE, Sch Math Sci, Lab Math & Complex Syst, Beijing 100875, Peoples R China
[4] Univ Stuttgart, Inst Algebra & Number Theory, Pfaffenwaldring 57, D-70569 Stuttgart, Germany