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Families omitting two values are chordally normal
Statement
Assume the Axiom of Choice. Let be a plane domain and let be a family of holomorphic functions omitting the two values and . Then is normal for chordal local uniform convergence.
Facts & Assumptions
Given: The Axiom of Choice, a plane domain , and a family whose members omit and .
The Axiom of Choice supplies the successive subsequence selections in the chordal Arzela-Ascoli criterion (The Axiom of Choice).
Schottky's theorem bounds such a function on every smaller disc once one of , , or is bounded at the center (Schottky's theorem).
A locally bounded holomorphic family is locally equicontinuous (Locally bounded holomorphic families are locally equicontinuous).
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
If is empty, it is chordally normal vacuously. Otherwise fix and choose with . For each , at least one of , , or is at most : if both and held, the triangle inequality would fail. Thus one of the three transforms , , has center value of modulus at most .
Each omits and , so [L1] applied after rescaling to gives a bound on 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 .
The target is compact, so pointwise relative compactness is automatic. Therefore [A1] and [L3] apply on each , giving chordal normality there. As was arbitrary, is chordally normal on .
Depends on
- The Axiom of Choice
- Families of holomorphic functions omitting two common finite values
- Chordal local uniform convergence and meromorphic normality
- Schottky's theorem
- Local chordal equicontinuity is equivalent to meromorphic normality on compact exhaustions
- Locally bounded holomorphic families are locally equicontinuous
Used by
Dependency tree · two levels
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Sources
- Aleksander Simonic, The Ahlfors lemma and Picard's theorems, Theorem 13 (standard reference, not scraped)