1. School of Mathematics and Statistics, Central South University 1 , Changsha 410083, People’s Republic of China
2. Department of Mathematics, Taiyuan University of Technology 2 , Taiyuan, Shanxi 030024, People’s Republic of China
3. School of Mathematics and Computational Science, Huaihua University 3 , Huaihua, Hunan 418008, People’s Republic of China