1. Centro Vito Volterra, Facultà di Economia, Università di Tor Vergata, Via di Tor Vergata, 00133 Roma, Italy
2. Nabeul Preparatory Engineering Institute, Mrezgua University Compus, 8000 Nabeul, Tunisia
3. Department of Mathematics, Faculty of Sciences of Tunis, University of Tunis El Manar, 1060 Tunis, Tunisia