1. Department of Mathematics, University of Sistan and Baluchestan, Zahedan, Iran
2. School of Sciences, Southwest Petroleum University, Chengdu, Sichuan 610500, P. R. China
3. Department of Electronic Engineering, City University of Hong Kong, Hong Kong SAR, P. R. China
4. School of Science, Southwest University of Science and Technology, Mianyang, Sichuan 621010, P. R. China
5. School of Mathematics and Statistics, Sichuan University of Science and Engineering, Zigong, Sichuan 643000, P. R. China