1. School of Mathematics and Computer Sciences, Yunnan Minzu University, Kunming 650504, China
2. School of Mathematics and Statistics, Yunnan University, Kunming 650031, China
3. College of Mathematics and Statistics, Qujing Normal University, Qujing, Yunnan 655011, China
4. Department of Mathematics, Henan Institute of Science and Technology, Xinxiang, Henan 453003, China