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HadwigerNelsonAtLeast4

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The "chromatic number of the plane" is at least 4. This can be proven by considering the [Moser-Spindel graph](https://de.wikipedia.org/wiki/Moser-Spindel) or the [Golomb graph](https://en.wikipedia.org/wiki/Golomb_graph) graph.

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Lean 4 Statement 
theorem HadwigerNelsonAtLeast4 : 4 ≤ χ(ℝ²)
ID: ErdosProblems__508__HadwigerNelsonAtLeast4
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