1. Department of Economics; University of Insubria; via Monte Generoso 71 21100 Varese Italy
2. Department of Information Engineering; Computer Science and Mathematics; University of L'Aquila; L'Aquila Italy
3. DEFAP; Università Cattolica del Sacro Cuore; Milan Italy
4. Department of Mathematics; Kwame Nkrumah University of Science and Technology; Kumasi Ghana