1. School of Mathematical Sciences, Laurent Mathematics Center and V.C. and V.R. Key Lab, Sichuan Normal University, Chengdu, People's Republic of China
2. Department of Basic Courses, Sichuan Vocational College of Finance and Economics, Chengdu, People's Republic of China
3. School of Mathematical Sciences, University of Electronic Science and Technology of China, Chengdu, People's Republic of China