this is an exercise I came across in Rudin's "Real and complex analysis" Chapter 16.

Suppose $\Omega$ is the complement set of $E$ in $\mathbb{C}$, where $E$ is a compact set with positive Lebesgue measure in the real line.

Does there exist a non-constant bounded holomorphic function on $\Omega$?

Especially, do this for $\Omega=[-1,1]$.

Some observations:

Suppose there exists such function $f$, then WLOG, we may assume $f$ has no zeros points in $\Omega$ by adding a large enough positive constant, then, $\Omega$ is simply-connected implies $\int_{\gamma}fdz=0$, for any closed curve $\gamma\subset \Omega$, how to deduce any contradiction?

  • $\begingroup$ Why do you say that $\Omega$ is simply connected? // You are going in the wrong direction: such a function exists on any domain with complement of positive measure on the line. Think of the Cauchy integral formula. // In the case of $E=[-1,1]$ (you have a typo there in the question) it's possible to find it explicitly using conformal maps. $\endgroup$ – user53153 Jan 3 '13 at 0:16
  • $\begingroup$ @Pavel M, the function constructed using Cauchy integral formula is not bounded on $\Omega$, this is the first several questions before this one on the book, you can check it. That is why Rudin post this question on the book. $\endgroup$ – ougao Jan 3 '13 at 0:23
  • $\begingroup$ Maybe it depends on how you use the Cauchy integral. Anyway, here's an example for $E=[-1,1]$: the conformal map $g(z)=\frac12(z+z^{-1})$ sends the unit disk onto the complement of $E$. Therefore, its inverse $f=g^{-1}$ is bounded by $1$ on the complement of $E$. // At least this shows that your attempt to find a contradiction could not work. $\endgroup$ – user53153 Jan 3 '13 at 0:37
  • $\begingroup$ You are correct, I double checked the problem, and found I have made a silly typo, in fact, I want to know whether there exists a non-constant bounded holomorphic(not just analytic) function, sorry for the confusion. $\endgroup$ – ougao Jan 3 '13 at 1:06
  • $\begingroup$ Does not change anything in my replies. I assumed you wanted a holomorphic function from the beginning. $\endgroup$ – user53153 Jan 3 '13 at 1:34

Reading Exercise 8 of Chapter 16, I imagine Rudin interrogating the reader.

Let $E\subset\mathbb R$ be a compact set of positive measure, let $\Omega=\mathbb C\setminus E$, and define $f(z)=\int_E \frac{dt}{t-z}$. Now answer me!
a) Is $f$ constant?
b) Can $f$ be extended to an entire function?
c) Does $zf(z)$ have a limit at $\infty$, and if so, what is it?
d) Is $\sqrt{f}$ holomorphic in $\Omega$?
e) Is $\operatorname{Re}f$ bounded in $\Omega$? (If yes, give a bound)
f) Is $\operatorname{Im}f$ bounded in $\Omega$? (If yes, give a bound)
g) What is $\int_\gamma f(z)\,dz$ if $\gamma$ is a positively oriented loop around $E$?
h) Does there exist a nonconstant bounded holomorphic function on $\Omega$?

Part h) appears to come out of the blue, especially since $f$ is not bounded: we found that in part (e). But it is part (f) that's relevant here: $\operatorname{Im}f$ is indeed bounded in $\Omega$ (Hint: write it as a real integral, notice that the integrand has constant sign, extend the region of integration to $\mathbb R$, and evaluate directly). Therefore, $f$ maps $\Omega$ to a horizontal strip. It's a standard exercise to map this strip onto a disk by some conformal map $g$, thus obtaining a bounded function $g\circ f$.

  • $\begingroup$ thanks, I should be more careful to deal with part (f). $\endgroup$ – ougao Jan 3 '13 at 14:20

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