1. 1Li Yang, School of Statistics and Mathematics, Yunnan University of Finance and Economics, Kunming 650221, China
2. 2Zhouhong Li, Department of Mathematics, Yuxi Normal University, Yuxi 653100, Yunnan, China
3. 3Liyan Pang, School of Mathematics and Computer Science, Ningxia Normal University, Guyuan, Ningxia 756000, China
4. 4Tianwei Zhang, City College, Kunming University of Science and Technology, Kunming 650051, China