1. Quaid-i-Azam University, Department of Mathematics, Islamabad, Pakistan + National University of Sciences and Technology, School of Natural Sciences, Department of Mathematics, Islamabad, Pakistan
2. Università degli Studi di Palermo, Dipartimento di Matematica e Informatica, Via Archirafi, Palermo, Italy
3. COMSATS Institute of Information Technology, Department of Mathematics, Attock, Pakistan
4. Quaid-i-Azam University, Department of Mathematics, Islamabad, Pakistan