Counting five-node subgraphs

09/23/2020

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by   Steve Lawford, et al.

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We propose exact count formulae for the 21 topologically distinct non-induced connected subgraphs on five nodes, in simple, unweighted and undirected graphs. We prove the main result using short and purely combinatorial arguments that can be adapted to derive count formulae for larger subgraphs. To illustrate, we give analytic results for some regular graphs, and present a short empirical application on real-world network data. We also discuss the well-known result that induced subgraph counts follow as linear combinations of non-induced counts.