1. Department of Mathematics, Computer Science and Economics, University of Basilicata, Campus Macchia Romana, Viale dell'Ateneo Lucano 10, 85100 Potenza, Italy
2. Departament de Física, Universitat Autònoma de Barcelona, 08193 Bellaterra, Catalonia, Spain
3. Institut d'Estudis Catalans, Carme 47, Barcelona 08001, Catalonia, Spain