1. International Department, Jiaying University, Meisong Road 100, Meizhou 514015, Guangdong, P. R. China
2. School of Mathematics, Sun Yat-sen University, Xingang West Road 135, Guangzhou 510275, Guangdong, P. R. China
3. Department of Mathematics, Guangzhou Foreign Language School, Fenghuang Road 102, Guangzhou 511455, Guangdong, P. R. China