1. School of Mathematics, Statistics and Computer Science , University of KwaZulu-Natal , Durban , South Africa
2. School of Mathematics, Statistics and Computer Science , University of KwaZulu-Natal , Durban , South Africa and Department of Mathematics , College of Natural and Computational Sciences , Addis Ababa Univeristy , Addis Ababa , Ethiopia
3. Department of Mathematics , Government College University , Katchery Road, Lahore 54000 , Pakistan