1. Centre d’économie de la Sorbonne, Université Paris 1, Panthéon Sorbonne, 106–112 Boulevard de l’Hôpital, 75647 Paris Cedex 13, France; and Department of Mathematics, College of Science, University El-Manar, Tunis, Tunisia
2. Centre d’économie de la Sorbonne, Université Paris 1, Panthéon Sorbonne, 106–112 Boulevard de l’Hôpital, 75647 Paris Cedex 13, France
3. Department of Mathematics, College of Science, King Saud University, Box 2455, Riyadh 11451, Saudi Arabia