Abstract

Understanding complex systems often requires a bottom-up approach, breaking the system into small and elementary constituents and mapping out the interactions between these components. In many cases, the myriads of components and interactions are best characterized as networks. For example, the society is a network of people connected by various links, including friendships (Milgram, 1967), collaborationships (Kochen, 1989; Wasserman & Faust, 1994), sexual contacts (Liljeros et al., 2001) or scientific co-authorships (Redner, 1998; Newman, 2001). Electronic communication relies on two very di#erent networks: the physical network wiring the routers together (Internet) (Faloutsos, Faloutsos & Faloutsos, 1999; Vazquez, Pastor-Satorras & Vespignani, 2002) and the web of homepages linked by URLs (World Wide Web) (Albert, Jeong & Barabasi, 1999; Lawrence & Giles, 1999; Broder et al., 2000). Airline, cell-phone, power-grid or business networks represent further examples of compl

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