1. School of Mathematics and Statistics, Liupanshui Normal University, Liupanshui, Guizhou, China
2. School of Mathematical Sciences, Sichuan Normal University, Chengdu, P.R. China
3. School of Economics and Management, Chongqing University of Arts and Sciences, Chongqing, China
4. School of Business, Sichuan Normal University, Chengdu, P.R. China