1. Department of Mathematics, Government College University, Faisalabad 38000, Pakistan
2. Mathematics, College of Engineering & Science, Victoria University, P. O. Box 14428, Melbourne City, MC 8001, Australia
3. Department of Mathematics, College of Science, King Saud University, Riyadh 11451, Saudi Arabia
4. Department of Mathematics, COMSATS University Islamabad, Islamabad 45540, Pakistan