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Locally injective k-colourings of planar graphs
Kratochvil, J.,Siggers, M. North Holland ; Elsevier Science Ltd 2014 Discrete Applied Mathematics Vol.173 No.-
A colouring of the vertices of a graph is called injective if every two distinct vertices connected by a path of length 2 receive different colours, and it is called locally injective if it is an injective proper colouring. We show that for k≥4, deciding the existence of a locally injective k-colouring, and of an injective k-colouring, are NP-complete problems even when restricted to planar graphs. It is known that every planar graph of maximum degree @?35k-52 allows a locally injective k-colouring. To compare the behaviour of planar and general graphs we show that for general graphs, deciding the existence of a locally injective k-colouring becomes NP-complete for graphs of maximum degree 2k (when k≥7).