1. College of Mathematics and Information Science, Henan Normal University, Xinxiang, People's Republic of China
2. School of Mathematics and Statistics, Xuchang University, Xuchang, People's Republic of China
3. School of Mathematical Sciences, Anhui University, Hefei, People's Republic of China
4. School of Electrical Engineering and Automation, Hefei University of Technology, Hefei, People's Republic of China