1. School of Science, Tianjin University of Commerce, Tianjin 300134, P. R. China
2. Tianjin Yi-Sheng Offshore Engineering Co., Ltd., Tianjin, China
3. Department of Mathematics, College of Science, Taif University, P. O. Box 11099, Taif 21944, Saudi Arabia
4. School of Mathematics and Statistics, Nanyang Normal University, Nanyang 473061, P. R. China