1. College of Science, National University of Defense Technology, Changsha, Hunan 410073, P. R. China
2. School of Mathematics, Changsha University, Changsha, Hunan 410022, P. R. China
3. Hunan Provincial Key Laboratory of Mathematical Modeling and Analysis in Engineering, Changsha University of Science and Technology, Changsha, Hunan 410114, P. R. China