1. Department of Applied Mathematics, School of Mathematics and Physics, University of Science and Technology Beijing, Beijing 100083, P. R. China
2. Hubei Key Laboratory of Mathematical Science, School of Mathematics and Statistics, Huazhong Normal University, Wuhan 430079, P. R. China
3. Laboratory of Mathematics and Complex Systems (BNU), School of Mathematical Sciences, Ministry of Education, Beijing Normal University, Beijing 100875, P. R. China