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This question already has an answer here:

Let $f$ be an entire function. Suppose that $f$ satisfies $$ |f(x+iy)|\leq\frac{1}{|y|}. $$ for all $x,y\in\mathbb{R}$. Prove that $f$ is identically zero.

I'm having some trouble with this, but I'm probably just overthinking it. The first instinct as usual is to try and make something happen with Liouville's theorem (and then since $f$ tends to zero on the imaginary axis we can conclude that $f\equiv 0$), but since $f$ is potentially unbounded on the real axis it's inapplicable. I'm pretty sure it's still impossible for $|f|\to\infty$ as $x\to\pm\infty$ while the function is bounded everywhere else, as it seems there would be some topological obstruction, but I could be wrong. I've also thought of getting an estimate on the $|f'|$ as in the proof of Liouville's theorem, but it fails for the same reasons. Any hints or tips are greatly appreciated. Thank you

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marked as duplicate by leo, user147263, Zev Chonoles, user99914, Claude Leibovici Jul 8 '15 at 4:54

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First approach: The first thing that comes to mind is Jensen's Formula.

For any point $\omega \in \mathbb{R}$ and $r > 0$, we have:

$$\log |f(\omega)| \le \frac{1}{2\pi}\int_0^{2\pi} \log |f(\omega + re^{i\theta})|\,d\theta$$

Since, $\displaystyle |f(\omega + re^{i\theta})| \le \frac{1}{|r\sin \theta|}$, we have:

$$\log |f(\omega)| \le -\frac{1}{2\pi}\int_0^{2\pi} \log |r\sin \theta|\,d\theta = - \log r - \frac{1}{2\pi}\int_0^{2\pi} \log |\sin \theta|\,d\theta$$

Since, the integral on the RHS is a constant,

$$|f(\omega)| \le \frac{K}{r}$$

Hence, $f(\omega)$ must be identically zero on $\mathbb{R}$ and hence on $\mathbb{C}$.


Second approach: Let us first fix a $r > 0$ and $z = re^{i\theta}$, then

$$|(z^2 - r^2)f(z)| = 2r|\Im(z).f(z)| \le 2r$$

Then, $g(\omega) = (\omega^2 - r^2)f(\omega)$ is holomorphic in the unit disc $D_r = \{\omega : |\omega| \le r \}$

Hence, by Maximum Modulus Principle:

$$|g(\omega)| \le 2r \textrm{ in the disc } D_r$$

and hence, $$|f(\omega)| < \frac{2r}{|\omega^2 -r^2|} \textrm{ in the interior of the disc }D_r$$

Letting $r \to \infty$, it follows that $f$ must be identically $0$.

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  • 1
    $\begingroup$ The first proof looks good, the second proof doesn't look right. We don't know $2iyf(z)$ is holomorphic, so how can you apply Cauchy to it? Also $a_{n-1} - |z|^2a_{n+1}$ is not a coefficient of a power series. However, you should be able to integrate $|2iyf(z)|^2$ on circles of radius $r$ to get the result. $\endgroup$ – zhw. Jul 6 '15 at 7:16
  • $\begingroup$ @zhw. you are right, the second approach needs tinkering. I'll improve it later. Thanks! :-) $\endgroup$ – r9m Jul 6 '15 at 7:54
  • $\begingroup$ For the first proof, is there any reasonable way to show the integral is finite? Also is using Lindelöf's theorem a possible method of proving this? $\endgroup$ – Blake Jul 6 '15 at 9:15
  • $\begingroup$ @Blake $\displaystyle \int_0^{2\pi} \log |\sin x| \,dx = -2\pi\log2$ :-) and as for Lindeloff Thm .. I don't see what motivations should I have for using that (although I admit the working principle are quite similar) $\endgroup$ – r9m Jul 6 '15 at 9:20
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    $\begingroup$ @r9m +1 for the first development. Just curious .... although it does not change the result, you left of the sum $$\sum_{n=1}^N\log \left(\frac{|z_n|}{r}\right)$$ where ${z_n}$ is the set of zeros of $f$ from Jensen. Was this tacitly omitted inasmuch as it simply has no impact on the result? $\endgroup$ – Mark Viola Jul 6 '15 at 17:05
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Here's another approach: Note that $z-\bar z=2iy.$ Thus

$$\tag 1 |(z-\bar z)f(z)| \le 2$$

for all $z.$ Let's write $f(z) = \sum_{n=0}^{\infty}a_nz^n.$ Setting $z=re^{it},$ we get

$$\tag 2 (z-\bar z)f(z) = (re^{it} - re^{-it})(\sum_{n=0}^{\infty}a_nr^ne^{int}).$$

Play with $(2)$ a little to see it equals

$$\tag 3 \sum_{n=0}^{\infty}(a_nr^{n+1} - a_{n+2}r^{n+3})e^{i(n+1)t} -a_0re^{-it} - a_1r^2.$$

Let's now integrate the modulus squared of $(3)$ over $[0,2\pi]$using the orthogonality of the exponentials. We get, recalling $(1),$

$$\sum_{n=0}^{\infty}|a_nr^{n+1} - a_{n+2}r^{n+3}|^2 +|a_0r|^2 +|a_1r^2|^2 \le 4.$$

This inequality holds for all $r>0.$ Verify that this holds iff $a_n=0$ for all $n,$ i.e., iff $f\equiv 0.$

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