The first theorem of graph theory tells us that the degree sum of a graph is two times the number of edges, or two times its size. A similar theorem is true in directed graph theory, or digraph theory. This theorem states that the sum of the outdegrees of the vertices in a digraph equals the sum of the indegrees, and they're both equal to m, the size of the digraph (the number of directed edges/arcs it has). We'll go over why this theorem is true in today's lesson!

Intro to directed graphs: [ Ссылка ]
First Theorem of Graph Theory: [ Ссылка ]

Thanks to Nasser Alhouti, Robert Rennie, Barbara Sharrock, and Lyndon for their generous support on Patreon!

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