[1] Univ Sci & Technol Beijing, Sch Math & Phys, Beijing 100083, Peoples R China
[2] Southwest Jiaotong Univ, Dept Math, Chengdu 611756, Sichuan, Peoples R China
[3] Jiangxi Normal Univ, Coll Math & Informat Sci, Nanchang 330022, Jiangxi, Peoples R China
[4] TU Kaiserslautern, FB Math, Postfach 3049, D-67653 Kaiserslautern, Germany
[5] Peking Univ, Sch Math Sci, Beijing 100871, Peoples R China