1. Division of Foundational Teaching, Guangzhou College of Technology and Business, Guangzhou 510850, China
2. Department of Mathematics, School of Science, ShenYang University of Technology, Shenyang 110870, China
3. Department of Mathematics, Yanbian University, Yanji 133001, China
4. School of Mathematics and Physics, Anqing Normal University, Anqing 246001, China