1. School of Science, Guangxi University of Science and Technology, Liuzhou, Guangxi 545006, P. R. China
2. Department of Mathematics, China University of Mining and Technology Beijing, Beijing 100083, P. R. China
3. Institute of Applied Physics and Computational Mathematics, P. O. Box 8009, Beijing 100088, P. R. China
4. Beijing Center for Mathematics and Information Interdisciplinary Sciences, Beijing 100088, P. R. China