Suppose $G$ is the simple graph with vertex set $\mathbb{R}^2$ created by connecting two points iff they are distance one from each other in the plane. How can we prove that $4\le \chi(G)\le7$? I think I have seen this problem somewhere before but can't pinpoint it. No hints please.
Unit distance graph in $\mathbb{R}^2$
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graph-theory
coloring
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1Wasn't this question just posted? – 2012-07-06
1 Answers
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The fact that there is a $4$-chromatic unit distance graph implies that $\chi \geq 4$, while the appropriate colouring of a hexagonal tiling shows that $\chi \leq 7$. For details, here's the Wikipedia article.