1. School of Mathematics and Statistics, Pingdingshan University, Pingdingshan, 467000, P.R. China. E-mail: zhaolaoshi_2010@163.com
2. Department of Mathematics and Information Science, Henan Normal University, Xinxiang, 453007, P.R. China. E-mails: yangxinguangyxg@163.com, cxn009@126.com
3. Department of Mathematics, China University of Mining and Technology, Xuzhou, 221008, P.R. China. E-mail: yanxj04@163.com