1. School of Mathematics and Computer Science, Yunnan Minzu University, Kunming 650500, China
2. School of Mathematics and Statistics, Yunnan University, Kunming 650091, China
3. School of Science, Nanjing University of Science and Technology, Nanjing 210094, China
4. School of Mathematics and Statistics, GuiZhou University of Finance and Economics, Guiyang 550025, China