1. University of Pisa , Department of Economics and Management , Via C. Ridolfi, 10, 56124 Pisa , Italy
2. University of Rome La Sapienza , Department of Mathematics , Piazzale Aldo Moro, 5, 00185 , Rome , Italy
3. University of Rome La Sapienza , Piazzale Aldo Moro, 5, 00185 , Rome , Italy