1. Department of Mathematics, Quaid-I-Azam University 45320, Islamabad 44000, Pakistan;
2. Department of Physics, Faculty of Sciences, King Saud University, P.O. Box 1846, Riyadh, Saudi Arabia
3. Department of Mathematics, Quaid-I-Azam University 45320, Islamabad 44000, Pakistan
4. Department of Mathematics, COMSATS Institute of Information Technology, H-8, Islamabad 44000, Pakistan