1. Department of Applied Mathematics, Lanzhou University of Technology, Lanzhou, Gansu, China
2. School of Mathematics and Physics, Jiangsu University of Technology, Changzhou, Jiangsu, China
3. Department of Mathematics, Zhejiang Normal University, Jinhua, Zhejiang, China
4. School of Mathematics and Statistics, Changsha University of Science and Technology, Changsha, Hunan, China