1. Department of Mathematics , Faculty of Sciences , 204390 Golestan University , Gorgan , Iran ; and School of Computer Science and Applied Mathematics, University of The Witwatersrand, 1 Jan Smuts Avenue, Braamfontein, Johannesburg, 2000, South Africa
2. Department of Mathematics , College of Science , 37850 King Saud University , P. O. Box 2455 , Riyadh 11451 , Saudi Arabia