1. School of Mechatronic Engineering, Lanzhou Jiaotong University, Lanzhou 730070, P. R. China
2. School of Mathematics and Statistics, Hexi University, Zhangye 734000, P. R. China
3. School of Mathematics and Physics, Lanzhou Jiaotong University, Lanzhou 730070, P. R. China
4. Institute for Complex Systems and Mathematical Biology Kings College, University of Aberdeen, Aberdeen AB24 3UE, UK