1. School of Mathematics, Thapar Institute of Engineering and Technology, Patiala 147004, India e-mail:
2. Department of Mathematics, M. S. Ramaiah University of Applied Sciences, Peenya Campus, Bengaluru 560058, India
3. Mathematics Department, University of Swaziland, Private Bag 4, Kwaluseni M201, Swaziland;
4. School of Mathematics, Statistics and Computer Science, University of KwaZulu-Natal, Private Bag X01, Scottsville, Pietermaritzburg 3209, South Africa