The time evolution of various properties of the PACS network: (a) the number of published papers, (b) the number of PACS codes, (c) the fraction of new and disappeared nodes, (d) the fraction of new and disappeared links, (e) the average degree, 〈k〉 and (f) the average link weight, 〈w〉.
The solid lines in (a), (e) and (f) denote a linear growth of 〈ΔNPapers〉 ≈ 508 papers per year, a yearly increase of 〈Δk〉 ≈ 0.44 of the average degree 〈k〉 and a yearly increase of 〈Δw〉 ≈ 0.02 of the average link weight 〈w〉, respectively. The solid line in (b) shows two roughly constant regimes, interspersed by a period of average linear increase of ΔNPACS = 13.5 PACS codes per year between 1990–2002. Note that k and w are heterogeneously distributed; see Fig. 2.