1. College of Mathematics and Statistics Qinghai Normal University Xining Qinghai People's Republic of China
2. The State Key Laboratory of Tibetan Intelligent Information Processing and Application Xining Qinghai People's Republic of China
3. Department of Mathematics Beijing Jiaotong University Beijing People's Republic of China
4. Faculty of Information Technology Macau University of Science and Technology Macau People's Republic of China
5. School of Computer Qinghai Normal University Xining Qinghai People's Republic of China
6. Department of Mathematics West Virginia University Morgantown West Virginia USA