1. School of Mathematical Sciences, Laurent Mathematics Center and V. C. & V. R. Key Lab, Sichuan Normal University, Chengdu 610066, P. R. China
2. School of Mathematical Sciences, Sichuan Normal University, Chengdu 610066, P. R. China
3. Department of Basic Courses, Sichuan Vocational College of Finance and Economics, Chengdu 610101, P. R. China
4. School of Mathematical Sciences, University of Electronic Science and Technology of China, Chengdu 611731, P. R. China