1. School of Mathematical Sciences, Qufu Normal University, Qufu, Shandong, 273165, China. longchunw@163.com
2. College of Mathematics and Statistics, Hunan Normal University, Changsha, Hunan, 410012, China. lankun.guo@gmail.com
3. School of Mathematics, Hunan University, Changsha, Hunan, 410082, China. liqingguoli@aliyun.com