[1] Sapienza Univ Roma, Dipartimento Matemat, I-00185 Rome, Italy
[2] Charles Univ Prague, KAM, CR-11800 Prague, Czech Republic
[3] Charles Univ Prague, ITI, CR-11800 Prague, Czech Republic
[4] CAMS EHESS, F-75013 Paris, France
[5] Univ Montpellier 2, Inst Math & Modelisat Montpellier, F-34095 Montpellier, France