[1] Politecn Milan, Dipartimento Matemat, Via Bonardi 9, I-20133 Milan, Italy
[2] Scuola Normale Super Pisa, Piazza Cavalieri 7, I-56126 Pisa, Italy
[3] Univ Grenoble, Inst Fourier, 100 Rue Maths, F-38402 St Martin Dheres, France
[4] Lab Fibonacci, Piazza Cavalieri 7, I-56126 Pisa, Italy
[5] Univ Napoli Federico II, Dipartimento Matemat & Applicaz, Via Cintia Monte S Angelo, I-80126 Naples, Italy
[6] Univ Trento, Dipartimento Matemat, Via Sommar 14, I-38123 Povo, TN, Italy