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A chordally locally uniform meromorphic limit is meromorphic or identically infinity
Statement
Let be a plane domain, and let be meromorphic functions converging chordally locally uniformly to a map . Then is meromorphic or identically . If every is holomorphic, then is holomorphic or identically .
Facts & Assumptions
Given: A plane domain and a chordally locally uniformly convergent sequence of meromorphic maps to .
Locally uniform limits of holomorphic functions are holomorphic (Locally uniform limits of holomorphic functions are holomorphic and their derivatives converge locally uniformly).
A locally uniform limit of nowhere-zero holomorphic functions is either identically zero or nowhere zero (Hurwitz's zero-free limit theorem).
Proof
A local uniform limit of continuous maps into the metric space is continuous, so is chordally continuous. If , choose a chordal neighbourhood of whose closure misses . On the chordal and Euclidean metrics are comparable, and continuity of together with chordal local uniform convergence gives a neighbourhood of with for all large . On the maps have no poles, hence are holomorphic there, and [L1] makes the Euclidean local uniform limit holomorphic near .
If , choose a chordal neighbourhood of whose complement is a closed Euclidean disc. Continuity of and chordal local uniform convergence give a neighbourhood of with for all large . The infinity-chart expressions and are then well-defined holomorphic maps on , and the same metric comparison turns into Euclidean local uniform convergence. Fact [L1] makes holomorphic, so is meromorphic at .
When every is holomorphic, the functions of step 1.2 are holomorphic and nowhere zero on . By [L2], their limit is either identically on or nowhere zero. Because , one gets on , so on . Thus the -value set of is open in the holomorphic-input case.
If takes some finite value, then steps 1.1 and 1.2 show that it is meromorphic at every point of ; otherwise . In the holomorphic-input case, the finite-value set is open by step 1.1 and the -value set is open by step 2.1, so connectedness leaves only the two possibilities: is holomorphic on all of , or .
Depends on
Used by
- The family e^(nz) converges chordally to infinity on the right half-plane without being holomorphically normal there Counterexample
- FALSE: a chordally locally uniform limit of holomorphic functions can never be identically infinity False statement
- Local chordal equicontinuity is equivalent to meromorphic normality on compact exhaustions Theorem
Dependency tree · two levels
18 results within two dependency steps of this one, each drawn at its shortest distance from it. An arrow runs from a result to what uses it, so the chart reads left to right and ends at this result, which carries a heavier outline. Every node is a link to that result. Click elsewhere on the chart to enlarge it.
Sources
- Matthias Weber, Complex Analysis, Ch. 5 §§5.1-5.2 (standard reference, not scraped)
- Elias M. Stein and Rami Shakarchi, Complex Analysis, Ch. 2 §5.2 and Ch. 8 §3.2 (standard reference, not scraped)
- Sheldon Axler, Paul Bourdon, and Wade Ramey, Harmonic Function Theory, Ch. 2 (standard reference, not scraped)