1. School of Mathematics, Faculty of Science, Beijing University of Technology, Beijing 100124, P. R. China
2. School of Mathematical Sciences, Capital Normal University, Beijing 100048, P. R. China
3. Department of Mathematics, North Carolina State University, Raleigh, NC 27695, USA
4. Interdisciplinary Research Institute, Faculty of Science, Beijing University of Technology, Beijing 100124, P. R. China