test if graph is a network
IsNetwork(G, s, t)
IsNetwork('G','s','t') outputs true if the directed graph G is a network with s as the source and t as the sink, and false otherwise.
IsNetwork('G') tests if a directed graph is a network. The output is a sequence of two sets of vertices, sources and sinks, of all possible sources and sinks. If these sets are both empty then G is implicitly not a network.
A network is a connected directed graph with at least one vertex with indegree 0 (the source) and at least one vertex with outdegree 0 (the sink). Note that under this definition a network may have an internal cycle.
N ≔ Digraph⁡1,2,1,3,2,4,3,4
N≔Graph 1: a directed graph with 4 vertices and 4 arc(s)
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