1. School of Mathematics and Statistics , Fuzhou University , Fuzhou, Fujian 350108 , China , E-mail:1070242634@qq.com
2. School of Mathematics and Statistics , Fuzhou University , Fuzhou, Fujian 350108 , China , E-mail: 411791320@qq.com
3. School of Mathematics and Statistics , Fuzhou University , Fuzhou, Fujian 350108 , China , E-mail: fdchen@263.net