Proof
(related to Corollary: Even Number of Vertices with an Odd Degree in Finite Digraphs)
 Let \(D=(V,E,\alpha,\omega)\) be a finite digraph and let \(O\subseteq V\) be the subset of all vertices with odd degree.
 Then it follows from the handshaking lemma that \[2E=\sum_{v\in V}d_D^+(v)+\sum_{v\in V}d_D^(v)=\underbrace{\sum_{v\in V}d_D(v)}_{\text{even}}.\]
 On the other hand, we have \[\sum_{v\in V}d_D(v)= \underbrace{\sum_{v\in V\setminus O}d_D(v)}_{\text{even}}+\underbrace{\sum_{v\in O}(d_D(v)1)}_{\text{even}}+O.\]
 Thus, \(O\) must be even, since it is a difference of even numbers.
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References
Bibliography
 Krumke S. O., Noltemeier H.: "Graphentheoretische Konzepte und Algorithmen", Teubner, 2005, 1st Edition