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Homomorphism duality for rooted oriented paths

Petra Smolíková — 2000

Commentationes Mathematicae Universitatis Carolinae

Let ( H , r ) be a fixed rooted digraph. The ( H , r ) -coloring problem is the problem of deciding for which rooted digraphs ( G , s ) there is a homomorphism f : G H which maps the vertex s to the vertex r . Let ( H , r ) be a rooted oriented path. In this case we characterize the nonexistence of such a homomorphism by the existence of a rooted oriented cycle ( C , q ) , which is homomorphic to ( G , s ) but not homomorphic to ( H , r ) . Such a property of the digraph ( H , r ) is called or * -. This extends the analogical result for unrooted oriented paths given in...

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