1. Department of Mathematics, Guangdong University of Education, Guangzhou, Guangdong, 510303, P.R. China
2. School of Mathematics and Statistics, Central South University, Changsha, Hunan, 410083, P.R. China.
3. College of General Education, Guangdong University of Science and Technology, Dongguan Guangdong, 523083, P.R. China.
4. School of Mathematics and Statistics, Hunan First Normal University, Changsha, Hunan, 410205, P.R. China.