Erdős problem 508 · problems.science

This upper bound for the chromatic number of the plane was observed by John R. Isbell. His approach was dividing the plane into hexagons of uniform size and coloring them with a repeating pattern. A proof can probably be found in:

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FormalConjectures/ErdosProblems/

508.lean

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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.

FormalConjectures/ErdosProblems/508.leanErdos508.HadwigerNelsonAtLeast41 lineExact file

14 ≤ (SimpleGraph.UnitDistancePlaneGraph Set.univ).chromaticNumber

SolvedStatement only, no proof

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