1. CRI, Mines ParisTech, 35 rue St Honoré, Fontainebleau, Seine-et-Marne, France
2. Department of Mathematics and Computer Science, Wesleyan University, 06459, MiddleTown, CT, USA
3. ETSI Informáticos, Universidad Politécnica de Madrid, Campus de Montegancedo s/n, 28660, Boadilla del Monte, Spain