1. University of Electronic Science and Technology of China, Zhongshan Institute, Zhongshan, Guangdong, People's Republic of China
2. School of Mathematics and Statistics, Lingnan normal University, Zhanjiang, Guangdong, People's Republic of China
3. Department of Mathematics, Shantou University, Shantou, Guangdong, People's Republic of China
4. Department of Mathematics, Jiaying University, Meizhou, Guangdong, People's Republic of China