Math CentralQuandaries & Queries


Question from Shabkhal, a parent:

Show that the sum of in degrees of all the nodes of a simple digraph is equal to the sum of out degrees of all its nodes and this sum is equal to the number of edges of the graph.

Hi Shabkhal,

Every arc has two ends. One end contributes to the out-degree of a vertex, and one contributes to the in-degree of a vertex.


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