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Uniform convergence on the closed unit disc does not give a holomorphic extension to a larger disc
Statement refuted
Refuted claim: If a complex power series converges uniformly on a closed disc, its sum extends holomorphically to some larger centred disc.
The series
converges uniformly on , but its sum on has no holomorphic extension to any disc centred at with radius greater than .
Facts & Assumptions
Given: The complex power series defining , with its partial sums and convergence interpreted as in Complex series, absolute convergence, complex power series, and radius of convergence.
If and the real series converges, then the complex function series converges absolutely pointwise and uniformly (Weierstrass M-test for complex-valued function series).
The series converges, while the harmonic series diverges (For rational , converges iff ).
Inside the radius of convergence, a complex power series may be differentiated term by term: (Inside its disc of convergence a complex power series is holomorphic and may be differentiated term by term).
A continuous real-valued function on a nonempty compact metric space is bounded and attains a maximum (A continuous real-valued function on a nonempty compact metric space is bounded and attains a greatest and a least value).
A complex differentiable function is continuous (Complex differentiability at a point implies continuity there).
Complex polynomials, including every monomial, are holomorphic (Complex polynomials are entire with the power-rule derivative, and rational functions are holomorphic wherever their denominator is nonzero).
Closed bounded subsets of Euclidean space, including the interval , are compact (Heine-Borel in : with the Euclidean metric a subset of is compact if and only if it is closed and bounded, and the proof by bisection uses no choice principle; the same holds on the real line).
Complex modulus is multiplicative and satisfies the triangle inequality, hence (Conjugation is an involutive real-field automorphism, , and modulus is definite, multiplicative, and subadditive).
Every holomorphic function has complex derivatives of every order locally (All higher complex derivatives exist and satisfy Cauchy's integral formula on an interior circle).
Counterexample
On , one has , so [L1] and the convergent series in [L2] give absolute pointwise and uniform convergence of the displayed series.
For real , [L3] gives .
Suppose, for contradiction, that a function holomorphic on for some agrees with on . By [L9], is holomorphic near , hence continuous by [L5]; [L8] makes continuous, [L7] makes compact, and [L4] bounds there.
Given , divergence in [L2] supplies with ; by [L6] and [L5], the finitely many monomials are continuous at , so choose with for every , and step 1.2 then gives .
Step 2.1 makes exceed every proposed bound for points , contradicting step 1.3; no such extension exists, and the refuted claim is false.
Depends on
- Weierstrass M-test for complex-valued function series
- For rational $p > 0$, $\sum 1/k^p$ converges iff $p > 1$
- Inside its disc of convergence a complex power series is holomorphic and may be differentiated term by term
- A continuous real-valued function on a nonempty compact metric space is bounded and attains a greatest and a least value
- Complex differentiability at a point implies continuity there
- Complex series, absolute convergence, complex power series, and radius of convergence
- Complex polynomials are entire with the power-rule derivative, and rational functions are holomorphic wherever their denominator is nonzero
- Heine-Borel in $\mathbb{R}^n$: with the Euclidean metric a subset of $\mathbb{R}^n$ is compact if and only if it is closed and bounded, and the proof by bisection uses no choice principle; the same holds on the real line
- Conjugation is an involutive real-field automorphism, $z\overline z=|z|^2$, and modulus is definite, multiplicative, and subadditive
- All higher complex derivatives exist and satisfy Cauchy's integral formula on an interior circle
Used by
Nothing in the library uses this result yet.
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Sources
- B. V. Shabat, Introduction to Complex Analysis, Remark 2.25 (standard reference, not scraped)