1. School of Mathematics and Statistics , HNP-LAMA , Central South University , Changsha , Hunan, 410083 , P. R. China
2. School of Mathematics and Statistics , HNP-LAMA , Central South University , Changsha , Hunan, 410083 ; and College of General Education, Guangdong University of Science and Technology, Dongguan, Guangdong, 523083 , P. R. China