Finding hidden links: Projection networks

A projection network is a particular type of network created from a bipartite network. A bipartite network, first introduced in Chapter 2, is one in which nodes can be categorised into two groups or partitions such that links only exist between the groups and never within either group. Examples of bipartite networks that have been analysed in the literature include: films and actors, where a link exists between an actor and a film they were in (see Newman, Strogatz and Watts, 2001; Tumminello et al, 2011); scientific papers and their authors (see Newman, 2001(a, b); Barabási et al, 2002); human genes and diseases (Goh et al, 2007); and the shareholder structure in Chinese companies (Pan et al, 2018). In finance, the two groups of nodes might represent investors and companies, with links representing investments: investors only invest in companies, so links only exist between the two groups. The corresponding projection network consists only of investors, and links exist between investors that invest in at least one common company (or, alternatively, the projection network could exist only of companies, in which case links would exist between companies that share at least one