1. Department of Mathematics, COMSATS University Islamabad, Islamabad, Lahore Campus, Pakistan
2. Department of Mathematics, University of Sargodha, Sargodha 40100, Pakistan
3. Department of Maths and Computer Science, University of Kinshasa, Democratic Republic of the Congo
4. Department of Mathematics and Statistics, College of Science, Taif University, P. O. Box 11099, Taif 21944, Saudi Arabia