1. School of Information Engineering, Ningxia University, Yinchuan, Ningxia 750021, China
2. School of Mathematics and Information Science, North Minzu University, Yinchuan, Ningxia 750021, China
3. School of Mathematics and Statistics, Hunan Normal University, Changsha, Hunan 410081, China
4. School of Electronic Information and Electrical Engineering, Chengdu University, Chengdu 610106, China