Statistical properties of e-mail and PGP networks.

N is the number of nodes of the giant component of the network considering only those links that are bidirectional (indicating mutual acquaintance between nodes). P(k) is the degree distribution, i.e., the histogram of the number of nodes with a give degree k (analytical expressions are best fits to the data using least squares method). ?C? is the average clustering coefficient and indicates the fraction of existing triangles between each node and its neighbours in the network, and r stands for the assortativity coefficient, that measures the tendency of nodes of high degree to link between them, r>0, or with nodes of lower degree, r[43].