Erdős problem 1119
Let be an infinite cardinal with . Let be a family of entire functions such that, for every , there are at most distinct values of . Must have cardinality at most ?
Sources
FormalConjectures/ErdosProblems/
1119.lean
Let be an infinite cardinal with . Let be a family of entire functions such that, for every , there are at most distinct values of . Must have cardinality at most ?
This is Problem 2.46 in [Ha74], where it is attributed to Erdős. The question is independent of ZFC, so the headline statement carries answer(sorry): it is neither provable nor refutable from the usual axioms of set theory.
The answer is yes if (see erdos_1119.variants.easy_case), so the question reduces to the case , where it is undecidable: Kumar and Shelah [KuSh17] produced a model of in which the answer is yes (with ), while Schilhan and Weinert [ScWe24] produced a different model of in which the answer is no.
True ↔ ∀ (m : Cardinal.{0}), Cardinal.aleph0 < m → m < Cardinal.continuum → ∀ (F : Set (ℂ → ℂ)), (∀ f ∈ F, Differentiable ℂ f) → (∀ (z₀ : ℂ), Cardinal.mk ↑{y | ∃ f ∈ F, f z₀ = y} ≤ m) → Cardinal.mk ↑F ≤ m