1. School of Mathematical Sciences, University of Science and Technology of China, Hefei, P. R. China
2. Faculty of Science, Jiangsu University, P. R. China
3. Department of Mathematics, COMSATS University Islamabad, Islamabad, Pakistan
4. National Engineering Laboratory for Modern Silk, Soochow University, Suzhou, P. R. China
5. Department of Computer Science, Bahria University Islamabad, Pakistan
6. Department of Mechanical Engineering, Sejong University, Seoul 143-747, Korea