1. School of Mathematics and Statistics, Huanghuai University, Zhumadian, Henan, P.R. China + School of Mathematical Sciences, Yangzhou University, Yangzhou, Jiangsu, P.R. China
2. College of Science, Beibu Gulf University, Qinzhou, Guangxi, P.R. China + School of Mathematical Sciences, Yangzhou University, Yangzhou, Jiangsu, P.R. China
3. School of Mathematical Sciences, Yangzhou University, Yangzhou, Jiangsu, P.R. China