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Erdős problem 1167

Erdős Problem 1167. Let r2r \geq 2 be finite, γ2\gamma \geq 2, and λ\lambda be an infinite cardinal. Let κα\kappa_\alpha be cardinals for all α<γ\alpha < \gamma. Is it true that 2λ(κα+1)α<γr+12^\lambda \to (\kappa_\alpha + 1)_{\alpha < \gamma}^{r+1} implies λ(κα)α<γr?\lambda \to (\kappa_\alpha)_{\alpha < \gamma}^r? Here ++ means cardinal addition, so that κα+1=κα\kappa_\alpha + 1 = \kappa_\alpha if κα\kappa_\alpha is infinite.

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FormalConjectures/ErdosProblems/

1167.lean

Retained formal statement3 of 8

The unrestricted version of Erdős Problem 1167 (without the condition γ2\gamma \geq 2 from the original Erdős–Hajnal list) is false. Taking γ=1\gamma = 1, κ0=1\kappa_0 = \aleph_1, λ=0\lambda = \aleph_0: the premise 20(1+1)132^{\aleph_0} \to (\aleph_1 + 1)^3_1 holds since 2012^{\aleph_0} \geq \aleph_1, but the conclusion 0(1)12\aleph_0 \to (\aleph_1)^2_1 fails since 0<1\aleph_0 < \aleph_1.

FormalConjectures/ErdosProblems/1167.leanErdos1167.erdos_1167.unrestricted_is_false7 linesExact file
¬∀ (r : ℕ),    2 ≤ r      ∀ (lam : Cardinal.{u}),        Cardinal.aleph0lam          ∀ (γ : Ordinal.{u}) (κ : γ.ToTypeCardinal.{u}),            (Combinatorics.cardinalPartitionRel (2 ^ lam) (r + 1) γ fun α => κ α + 1) →              Combinatorics.cardinalPartitionRel lam r γ κ
TestStatement only, no proof

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