1. School of Mathematical Sciences, University of Electronic Science and Technology of China, Chengdu, Sichuan, P.R. China;
2. School of Economic Mathematics/Institute of Mathematics, Southwestern University of Finance and Economics, Chengdu, Sichuan, P.R. China;
3. School of Mathematics and Computational Science, Hunan University of Arts and Science, Changde, Hunan, P.R. China