1. Department of Mathematics, Nanchang University, Nanchang, Jiangxi 330031, P.R. China. E-mails: cjh19881129@163.com, xjhuangxwen@163.com
2. School of Mathematics and Statistics, Central South University, Changsha, Hunnan 410083, P.R. China. E-mail: qindd132@163.com
3. School of Mathematics and Statistics, Qujing Normal University, Qujing, Yunnan 655011, P.R. China. E-mail: chengbitao2006@126.com