[1] Univ Paris 11, CNRS, LPT, UMR 8627, Orsay, France
[2] ISI Fdn, Complex Networks Lagrange Lab, Turin, Italy
[3] CNR, Ist Sistemi Complessi, Florence, Italy
[4] Univ Roma La Sapienza, Dipartimento Fis, Ctr SMC, CNR INFM, I-00185 Rome, Italy
[5] Ctr Study Complex Networks, Linkalab, Cagliari, Italy
[6] Univ Cagliari, SLACS CNR INFM, Dipartimento Fis, Cagliari, Italy
[7] Ist Nazl Fis Nucl, I-95129 Catania, Italy
[8] Univ Catania, Dipartimento Fis & Astron, Catania, Italy
[9] Northwestern Univ, Dept Phys & Astron, Evanston, IL USA