1. Department of Economics, Social Studies, Applied Mathematics, and Statistics, School of Management and Economics, University of Torino, 10134 Torino, Italy, and Collegio Carlo Alberto, 10122 Torino, Italy.
2. Department of Mathematics, Uppsala University, 75105 Uppsala, Sweden.
3. Department of Mathematics and Mathematical Statistics, Umeå University, 90187 Umeå, Sweden.