1. Dipartimento di Matematica e Informatica, Università degli Studi di Catania, Catania, Italy
2. Department of Mathematics and Applied Mathematics, University of Johannesburg, Auckland Park, South Africa
3. National Institute for Theoretical and Computational Sciences (NITheCS), South Africa
4. Dipartimento di Scienze Mathematiche, Informatiche e Fisiche, University of Udine, Udine, Italy