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On good characterizations for digraphs. (English) Zbl 0609.05040
For any digraph D let f(D) denote the set of all digraphs G such that there is not homomorphism from A to G and let g(D) denote the set of all digraphs H such that there is a homomorphism from H to D. The author gives new examples of digraphs A and B such that $$f(A)=g(B)$$ and he gives a necessary condition that must be satisfied by such digraphs A.
Reviewer: J.W.Moon
##### MSC:
 05C20 Directed graphs (digraphs), tournaments
##### Keywords:
directed graphs; digraphs; homomorphism
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