1. Faculty of Science, Jiangsu University, Zhenjiang, Jiangsu 212013, P. R. China
2. Department of Mathematics, COMSATS University, Islamabad 45550, Pakistan
3. National Engineering Laboratory for Modern Silk, Soochow University, Suzhou 215123, P. R. China
4. Department of Mathematics, University of Science and Technology, Hefei, Anhui 230026, P. R. China