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