Density of 5/2-critical graphs


A graph G is 5/2-critical if G has no circular 5/2-coloring (or equiva-lently, homomorphism to C5), but every proper subgraph of G has one. We prove that every 5/2-critical graph on n ≥ 4 vertices has at least 5n−24 edges, and list all 5/2-critical graphs achieving this bound. This implies that every planar or projective-planar graph of girth at least 10 is 5/2-colorable.

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oaioai:CiteSeerX.psu:10.1...Last time updated on 10/30/2017

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