1. MOE-LCSM, School of Mathematics and Statistics, Hunan Normal University, Changsha, Hunan 410081, P. R. China
2. School of Science, Beijing University of Posts and Telecommunications, Beijing 100876, P. R. China
3. Key Laboratory of Wu Wen-Tsun Mathematics, CAS, School of Mathematical Science, University of Science and Technology of China, Hefei 230026, P. R. China