1. Centro Vito Volterra, Facoltà di Economia, Università di Roma Tor Vergata, Via di Tor Vergata, 00133 Roma, Italy
2. International Institute for Advanced Studies, 9-3 Kizugawadai, Kizu, Soraku-gun, Kyoto 619-0225, Japan
3. Department of Mathematics, Louisiana State University, Baton Rouge, LA 70803-4918, USA