1. Department of Mathematics, Chongqing Jiaotong University, Chongqing, People's Republic of China
2. School of Economics and Management, Chongqing Jiaotong University, Chongqing, People's Republic of China
3. School of Information, Renmin University of China, Beijing, People's Republic of China
4. School of Computer and Communication Engineering, University of Science and Technology Beijing, Beijing, People's Republic of China