Affiliation: | 1. School of Mathematics, Northwest University, Xi'an, P. R. Chinachina-lizhiming@163.com;3. School of Mathematics, Northwest University, Xi'an, P. R. China;4. Department of Mathematics &5. Computer Science, University of North Carolina at Pembroke, Pembroke, USA;6. School of Management Science and Engineering, Nanjing University of Information Science &7. Technology, Nanjing, Jiangsu, P. R. China |