1. Scuola Normale Superiore di Pisa, Piazza dei Cavalieri, Pisa 56126, Italy
2. Chair of Econophysics and Complex Systems, Ecole polytechnique, Palaiseau Cedex 91128, France
3. Department of Mathematics, Università di Bologna, Piazza di Porta San Donato 5, Bologna 40126, Italy
4. CADS, Human Technopole, Milan, Italy
5. Capital Fund Management, 23 rue de l'Université, Paris 75007, France
6. CFM-Imperial Institute of Quantitative Finance, Department of Mathematics, Imperial College, 180 Queen's Gate, SW7 2RH London, UK
7. Ladhyx UMR CNRS 7646 & Department of Economics, Ecole Polytechnique, Palaiseau Cedex 91128, France