1. Department of Physics, University of Trieste, Via Valerio 2, 34127 Trieste, Italy
2. Department of Mathematics, Amity University, Major Arterial Road, Action Area II, Rajarhat, New Town, Kolkata 700135, India
3. Department of Mathematical Sciences, University of Zululand, Private Bag X1001, Kwa-Dlangezwa 3886, South Africa