Erdős problem 918 · problems.science

A question of Erdős and Hajnal [ErHa68b], who proved that for every finite $k$ there is a graph with chromatic number $\aleph_1$ and $\aleph_k$ vertices where each subgraph on less than $\aleph_k$ vertices has chromatic number $\leq \aleph_0$ .

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9 retained statements2415f78e850a

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FormalConjectures/ErdosProblems/

918.lean

Retained formal statement7 of 9

In [ErHa69] the questions are stated with $= \aleph_0$ rather than $\leq\aleph_0$ . This is a likely typo since it can be shown that no such graph exists in this case.

This is the first question with induced subgraphs.

FormalConjectures/ErdosProblems/918.leanErdos918.erdos_918.variants.eq_aleph_0.parts.i4 linesExact file

1¬∃ V G,2    Cardinal.mk V = Cardinal.aleph 2 ∧3      G.chromaticCardinal = Cardinal.aleph 2 ∧4        ∀ (W : Set V), Cardinal.mk ↑W = Cardinal.aleph 1 → (SimpleGraph.induce W G).chromaticCardinal = Cardinal.aleph0

TextbookStatement only, no proof

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