1. Semeion Research Center of Sciences of Communication, 00128 Rome, Italy
2. Department of Mathematical and Statistical Sciences, University of Colorado, Denver, CO 80204, USA
3. Department of Civil, Construction-Architectural and Environmental Engineering, University of L’Aquila, 67100 L’Aquila, Italy
4. Department of Engineering, University of Perugia, 06123 Perugia, Italy