[1] Univ Padua, Dipartimento Matemat Tullio Levi Civita, Via Trieste 63, I-35121 Padua, Italy
[2] Univ Lisbon, Fac Ciencias, Ctr Matemat Aplicacoes Fundamentais & Invest Oper, Bloco C6,Piso 2, P-1749016 Lisbon, Portugal
[3] Univ Lisbon, Fac Ciencias, Dept Matemat, Bloco C6,Piso 2, P-1749016 Lisbon, Portugal
[4] Ecole Polytech, Inst Polytech Paris, CNRS, CMLS, F-91128 Palaiseau, France