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LemmaStatement: Literature-sourcedProof: AI-adaptedPipeline-generatedprecheck passjudge pass (gpt-6.1-sol)audited 2026-10-02
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Oka-Weil approximation on a domain of holomorphy (host-domain lemma)

Statement

Assume the Axiom of Choice (AC). Let D⊆Cn be a domain of holomorphy (Holomorphic extension and domains of holomorphy in several variables), let K⋐D be compact and convex with respect to the holomorphic functions on D, i.e. K^D=K (Holomorphic hulls and holomorphic convexity), and let f be holomorphic in an open neighbourhood of K. Then for every ε>0 there is F∈O(D) with sup⁡z∈K∣F(z)−f(z)∣<ε.

Facts & Assumptions

Given: The Axiom of Choice; a domain D⊆Cn of holomorphy; a compact K⋐D with K^D=K; a function f holomorphic on an open neighbourhood of K; and ε>0.

[F1]

The holomorphic hull is E^D:={a∈D:∣g(a)∣≤sup⁡z∈E∣g(z)∣ for every g∈O(D)} (Holomorphic hulls and holomorphic convexity). Thus K^D=K is exactly the compact O(D)-convexity hypothesis.

[F2]

(Harold P. Boas, Lecture Notes on Multidimensional Complex Analysis, §3.3.2, Theorem 21, printed p. 79, with proof on printed pp. 79–80.) If G is a domain of holomorphy in Cn and K0⋐G is compact and convex with respect to O(G), then every function holomorphic in a neighbourhood of K0 is uniformly approximable on K0 by functions in O(G). The proof uses finite analytic-polyhedron reduction, Oka's graph lift and a ∂ˉ correction, a power-series approximation, and a telescoping exhaustion.

[F3]

The Axiom of Choice supplies a choice function for every family of nonempty sets (The Axiom of Choice).

Choice use. AC is the ambient hypothesis recorded in the Statement and cited as [F3]. No additional choice is made in applying [F2].

Proof

technique · direct application of the cited approximation theorem
1.1F1F2given

If K=∅, take F=0, since the supremum of the nonnegative empty family is 0 in the convention of [F1]. Otherwise the hypotheses of Boas's Theorem 21 [F2] hold with G:=D and K0:=K: D is a domain of holomorphy, and K^D=K is the required O(D)-convexity condition by [F1]. The given f is holomorphic in a neighbourhood of K.

2.1F2F3step 1.1given∎

For nonempty K, apply [F2] with approximation tolerance ε. It gives F∈O(D) with sup⁡z∈K∣F(z)−f(z)∣<ε, which is the Statement under the ambient AC assumption [F3].

Depends on

Used by

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Sources