1. Department of Applied Mathematics, Nanjing Forestry University, Nanjing 210037, P. R. China
2. Department of Mathematics, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia
3. Department of Mathematics, Quaid-i-Azam University, Islamabad 44000, Pakistan
4. Department of Mathematics, Inha University, Incheon 402-751, Korea
5. School of Mathematics, Shanghai University of Finance and Economics, Shanghai 200433, P. R. China