4.4 Prove Let D be a nontrivial connected digraph. Then D is Eulerian if and only if
od(v)=id(v) for every vertex v of D.
Od means the outdegree of a vertex v of a digraph D. (is the number of vertices of D that are adjacent from v.
id means the indegree of a vertex v of a digraph D.( is the number of vertices of D adjacent to v.
This is a proof regarding Eulerian nontrivial connected digraphs.