1. School of Computer, Qinghai Normal University, Xining 810008, Qinghai, China
2. School of Mathematics and Statistics, Qinghai Normal University, Xining 810008, Qinghai, China
3. Academy of Plateau Science and Sustainability, People's Government of Qinghai Province and Beijing Normal University, China
4. Department of Mathematics, Beijing Jiaotong University, Beijing 100044, China