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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FormalConjectures/ErdosProblems/

918.lean

Retained formal statement3 of 9

Is there a graph with $\aleph_2$ vertices and chromatic number $\aleph_2$ such that every subgraph on $\aleph_1$ vertices has chromatic number $\leq\aleph_0$ ?

FormalConjectures/ErdosProblems/918.leanErdos918.erdos_918.variants.all_subgraphs.parts.i5 linesExact file

1True ↔2  ∃ V G,3    Cardinal.mk V = Cardinal.aleph 2 ∧4      G.chromaticCardinal = Cardinal.aleph 2 ∧5        ∀ (H : G.Subgraph), Cardinal.mk ↑H.verts = Cardinal.aleph 1 → H.coe.chromaticCardinal ≤ Cardinal.aleph0

OpenStatement only, no proof

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