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In page Network motif:

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In addition, three specific concepts of sub-graph frequency have been proposed.[11] As the figure illustrates, the first frequency concept F1 considers all matches of a graph in original network. This definition is similar to what we have introduced above. The second concept F2 is defined as the maximum number of edge-disjoint instances of a given graph in original network. And finally, the frequency concept F3 entails matches with disjoint edges and nodes. Therefore, the two concepts F2 and F3 restrict the usage of elements of the graph, and as can be inferred, the frequency of a sub-graph declines by imposing restrictions on network element usage. As a result, a network motif detection algorithm would pass over more candidate sub-graphs if we insist on frequency concepts F2 and F3.