[1] Univ Padua, Dipartimento Matemat Tullio Levi Civita, Via Trieste 63, I-35121 Padua, Italy
[2] Ctr Matemat & Aplicacoes Fundamentais, Ctr Invest Operac, Edificio C 6,Piso 2, P-1700 Lisbon, Portugal
[3] FCUL, Dept Matemat, Edificio C 6,Piso 2, P-1700 Lisbon, Portugal
[4] CNRS, Ecole Polytech, Inst Polytech Paris, CMLS, F-91128 Palaiseau, France