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TheoremStatement: Literature-sourcedProof: AI-generatedprecheck passaudited 2026-08-30
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Families omitting two values are chordally normal

Statement

Assume the Axiom of Choice. Let Ω be a plane domain and let F⊆H(Ω) be a family of holomorphic functions omitting the two values 0 and 1. Then F is normal for chordal local uniform convergence.

Facts & Assumptions

Given: The Axiom of Choice, a plane domain Ω, and a family F⊆H(Ω) whose members omit 0 and 1.

[A1]

The Axiom of Choice supplies the successive subsequence selections in the chordal Arzela-Ascoli criterion (The Axiom of Choice).

[L1]

Schottky's theorem bounds such a function on every smaller disc once one of ∣f(a)∣, ∣1/f(a)∣, or ∣1/(1−f(a))∣ is bounded at the center (Schottky's theorem).

[L2]

A locally bounded holomorphic family is locally equicontinuous (Locally bounded holomorphic families are locally equicontinuous).

[L3]

Under the Axiom of Choice, the chordal Arzela-Ascoli criterion characterizes meromorphic normality (Local chordal equicontinuity is equivalent to meromorphic normality on compact exhaustions).

Proof

technique · direct
1.1givencasesalgebra

If F is empty, it is chordally normal vacuously. Otherwise fix a∈Ω and choose ρ>0 with D(a,2ρ)‾⊂Ω. For each f∈F, at least one of ∣f(a)∣, ∣1/f(a)∣, or ∣1/(1−f(a))∣ is at most 2: if both ∣f(a)∣<1/2 and ∣1−f(a)∣<1/2 held, the triangle inequality would fail. Thus one of the three transforms T1(z)=z, T2(z)=1/z, T3(z)=1/(1−z) has center value of modulus at most 2.

2.1L1L2step 1.1algebra

Each Tj∘f omits 0 and 1, so [L1] applied after rescaling D(a,2ρ) to D gives a bound ∣Tj(f(z))∣≤M(ρ) on D(a,ρ) for the transform selected in step 1.1. Hence every member of the transformed family is locally bounded there. Fact [L2] makes each transformed subfamily locally equicontinuous, and because there are only three fixed inverse transforms, the original family is chordally locally equicontinuous on D(a,ρ).

3.1A1L3step 2.1∎

The target C^ is compact, so pointwise relative compactness is automatic. Therefore [A1] and [L3] apply on each D(a,ρ), giving chordal normality there. As a was arbitrary, F is chordally normal on Ω.

Depends on

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