[1] South China Univ Technol, Dept Math, Guangzhou 510640, Guangdong, Peoples R China
[2] Tsinghua Univ, Dept Math Sci, Beijing 100084, Peoples R China
[3] Capital Normal Univ, Sch Math Sci, Beijing 100048, Peoples R China
[4] Max Planck Inst Math Sci, D-04103 Leipzig, Germany