1. School of Mathematics and Physics, Southwest University of Science and Technology, Mianyang, Sichuan 621010, P. R. China
2. Sino-British Collaborative Education, Chengdu University of Technology, Chengdu, Sichuan 610059, P. R. China
3. School of Science, Civil Aviation Flight University of China, Guanghan, Sichuan 618307, P. R. China