1. School of Mathematics and Statistics, Northwest Normal University, Lanzhou, China
2. Gansu Provincial Research Center for Basic Disciplines of Mathematics and Statistics, Lanzhou, China
3. School of Decision Science, The Hang Seng University of Hong Kong, Shatin, Hong Kong
4. Department of Mathematics and Information Technology, The Education University of Hong Kong, Tai Po, Hong Kong
5. School of Computer Science, University of Hull, United Kingdom