1. The Key Laboratory of Intelligent Information and Data Processing of NingXia Province, North Minzu University, Yinchuan, People's Republic of China
2. School of Mathematical Sciences, Tianjin Polytechnic University, Tianjin, People's Republic of China
3. Department of Mathematics, Zhejiang Normal University, Jinhua, People's Republic of China
4. College of Science, Civil Aviation University of China, Tianjin, People's Republic of China