1. School of Applied Mathematics, Guangdong University of Technology, Guangzhou, 510006, P. R. China
2. School of Mathematics and Statistics, Yunnan University, Kunming, Yunnan, 650031, P. R. China
3. Department of Mathematics, Changshu Institute of Technology, Changshu, Jiangsu, 215500, P. R. China
4. Lingnan College and School of Mathematics, Sun Yat-sen University, Guangzhou, 510275, P. R. China