This manuscript was prepared as a fuller and more accessible account of the ideas presented in Guimerá and Amaral (2005 Nature 433 895-900), which is included as  in the manuscript. In hindsight, this should have been made clearer in the original submission. We sincerely apologize for any confusion this oversight might have caused.
ASJC Scopus subject areas
- Statistical and Nonlinear Physics
- Statistics and Probability
- Statistics, Probability and Uncertainty