1. Departamento de Matematicas, Universidad de Cádiz, Puerto Real, 11510 Cádiz, Spain
2. Dipartimento di Matematica e Informatica, Università di Catania, 95125 Catania, Italy
3. Department of Mathematical Sciences, International Institute for Symmetry Analysis and Mathematical Modelling, North-West University, Mafikeng Campus, Mmabatho 2735, South Africa