Planar Ramsey graphs

  title={Planar Ramsey graphs},
  author={Maria Axenovich and Carsten Thomassen and Ursula Schade and Torsten Ueckerdt},
  journal={Electron. J. Comb.},
We say that a graph $H$ is planar unavoidable if there is a planar graph $G$ such that any red/blue coloring of the edges of $G$ contains a monochromatic copy of $H$, otherwise we say that $H$ is planar avoidable. That is, $H$ is planar unavoidable if there is a Ramsey graph for $H$ that is planar. It follows from the Four-Color Theorem and a result of Gonçalves that if a graph is planar unavoidable then it is bipartite and outerplanar. We prove that the cycle on $4$ vertices and any path are… 

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