  A key ingredient of current models proposed to capture the topological
evolution of complex networks is the hypothesis that highly connected nodes
increase their connectivity faster than their less connected peers, a
phenomenon called preferential attachment. Measurements on four networks,
namely the science citation network, Internet, actor collaboration and science
coauthorship network indicate that the rate at which nodes acquire links
depends on the node's degree, offering direct quantitative support for the
presence of preferential attachment. We find that for the first two systems the
attachment rate depends linearly on the node degree, while for the latter two
the dependence follows a sublinear power law.
