1. Centro Vito Volterra, Facultà di Economia, Università di Tor Vergata, Via di Tor Vergata, 00133 Roma, Italy
2. Department of Mathematics, Faculty of Sciences of Tunis, University of Tunis El-Manar, 1060 Tunis, Tunisia
3. Department of Mathematics, Higher Institute of Applied Science and Technology of Gabes, University of Gabes, Tunisia