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