Complexity of locally-injective homomorphisms to tournaments

10/24/2017

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by   Stefan Bard, et al.

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For oriented graphs G and H, a homomorphism f: G → H is locally-injective if, for every v ∈ V(G), it is injective when restricted to some combination of the in-neighbourhood and out-neighbourhood of v. Two of the possible definitions of local-injectivity are examined. In each case it is shown that the associated homomorphism problem is NP-complete when H is a reflexive tournament on three or more vertices with a loop at every vertex, and Polynomial when H is a reflexive tournament on two or fewer vertices.