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Is there a characterization of the nonconstant entire functions $f$ that satisfy $|f(z)|=1$ for all $|z|=1$?

Clearly, $f(z)=z^n$ works for all $n$. Also, it's not difficult to show that if $f$ is such an entire function, then $f$ must vanish somewhere inside the unit disk. What else can be said about those functions?

Thank you

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6 Answers 6

The only such functions are $f(z)=az^n$ with $|a|=1$. Let $f$ be such a function and define $$g(z)=\overline{f(\overline{z}^{-1})}^{-1}.$$ Due to the two complex conjugations, $g$ is meromorphic on $\mathbb{C}-\{0\}$. Also $g(z)=f(z)$ when $|z|=1$. By analytic continuation, $f(z)=g(z)$ for all nonzero $z$. Now $g$ has a pole of order $k$ at $\infty$ where $k$ is the order of $f$ at zero. Hence $f$ is a polynomial of degree $k$. As $k$ is also the order at $0$ then $f(z)=az^k$; clearly $|a|=1$.

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I think You implicitly assumed that $f(z) = 0\iff z = 0$ ($g$ has a pole in $\bar{a}^{-1}$ if $f(a) = 0$), but in this case the claim follows almost immediately from maximum modulus principle. –  ifk Aug 27 '10 at 9:31
    
Thanks, now corrected. –  Robin Chapman Aug 27 '10 at 16:55
    
Pierre-Yves, an entire function with a pole (i.e., not an essential singularity) at $\infty$ s a polynomial. –  Robin Chapman Aug 28 '10 at 13:13
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Pierre-Yves, $f$ is entire, since that is the hypothesis! –  Robin Chapman Aug 28 '10 at 13:59
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This must be from the Book. –  timur Dec 24 '10 at 6:14
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Edit: How frustrating, this is still not correct. It is not hard to make this proof work by generalizing to the functions holomorphic on unit disc and continuous on its closure, but such approach will be nothing else than proving the theorem cited by Jonas Meyer (see his answer).
Second attempt:

  1. Note that $f$ has finitely many zeros in $\mathbb{D}$ (identity principle + compactness).

  2. Observe that if $F\in \mathcal{O}(\mathbb{D})\cap \mathcal{C}(\bar{\mathbb{D}}),$ the only zero of $F$ is in zero, and $|F|$ is constant on unit circle, then $F(z) = e^{it}z^{k}$ where $k = ord_{0}F$.
    (apply the maximum principle for $F(z)/z^{k}$ and $z^{k}/F(z)$)

  3. Let $z_{1},\ldots, z_{n}$ be all pairwise distinct zeros of $f$ in $\mathbb{D}$. By induction on $n$ we will show that $n = 1$ and $z_{1} = 0$.

Let $n = 1$.
We are taking $h \in Aut(\mathbb{D})$ s.t. $h(0) = z_{1}$ and $g:= f\circ h\in \mathcal{O}(\mathbb{D})\cap \mathcal{C}(\bar{\mathbb{D}})$.
Then $|g|$ is constant on unit circle, $0$ is the only zero of $g$ in $\mathbb{D}$. So $g(z) = e^{it}z^{k}$.
Therefore $f(z) = e^{it}\cdot \left(\frac{z - z_{1}}{1 - \bar{z_{1}}z}\right)^{k}$ in $\mathbb{D}$.
From identity principle for meromorphic functions this equality holds in $\mathbb{C}$, and gives contradiction unless $z_{1} = 0$ (otherwise $f$ is not entire).

If $n > 1$, then we're reasoning by contradiction.
W.l.o.g. $z_{n}\neq 0$.
We are taking $h \in Aut(\mathbb{D})$ s.t. $h(0) = z_{n}$ and $g:= f\circ h\in \mathcal{O}(\mathbb{D})\cap \mathcal{C}(\bar{\mathbb{D}})$, and then $h^{-1}(z_{1}),\ldots, h^{-1}(z_{n-1}), 0$ are the all zeros of $g$ in $\mathbb{D}$.
Let $k = ord_{0}g$, and $G(z) = g(z)/z^{k}\in \mathcal{O}(\mathbb{D})\cap \mathcal{C}(\bar{\mathbb{D}})$.
$G$ has $n-1$ zeros on $\mathbb{D}$, $|G(z)| = 1$ on unit circle, thus from inductive assumption $n-1 = 1$ or $G$ is constant (we cannot use inductive assumption since in general $G\not\in \mathcal{O}(\mathbb{C})$!).
If $G$ is non-constant then $n-1 = 1$ and $G(z) = e^{it}z^{l}, \ l > 0$ - a contradiction with $ord_{0}g = k$. Otherwise $n - 1 = 0$ - a contradiction.

Finally 1. and 3. proves that all entire functions with desired property are of the form $f(z) = e^{it}z^{k}$.

The following reasoning is not correct (see the Malik Younsi's answer):

This should be the comment to the Leandro's answer, although I cannot add comments yet.

W.l.o.g. $f$ is non-constant. Then by maximum modulus $f$ has zero inside $\mathbb{D}$. Denote this zero by $a$.

Composing $f$ with an automorphism $h$ of disc s.t. $h(0) = a$ we obtain that $g = f\circ h$ satisfies $g(0)= 0$ and moreover $|g(z)|\le |z|^{k}$ inside $\mathbb{D}$, where $k$ is the order of zero of $g$ in $0$ (indeed $g(z)/z^{k}$ is entire and we may use the maximum modulus principle).

By Schwarz lemma there exists $\theta\in \mathbb{R}$ satisfying $g(z) = e^{i\theta}z^{k}$.

From classification of automorphisms of the disc, we have $h^{-1}(z) = e^{i\omega}\frac{z-a}{1-\bar{a}z}$ for some $\omega\in \mathbb{R}$, and from this the full classification easily follows.

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Hi ifk, this is in fact a complement to my partial answer :), nice! –  Leandro Aug 26 '10 at 21:02
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If a function $f$ is holomorphic in a neighborhood of the closed disk and has modulus 1 on the circle, then $f$ is a finite Blaschke product. You can prove this by taking all of the zeros inside the disk counted according to multiplicity, dividing by corresponding holomorphic automorphisms of the disk that have those zeros, and showing that the result is constant. (This quotient and its reciprocal are analytic and bounded by 1 on the disk...) Thus, your $f$ is a finite Blaschke product, but it also has no poles in the plane. The only such functions are constant multiples of power functions, as ifk already said.

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Nice answer. In fact, my proof was (again) flawed, but one can modify it to give alternative proof of this formula with finite Blaschke product. –  ifk Aug 27 '10 at 10:15
    
Nice, the solution is easy once you use Blaschke products! –  Malik Younsi Aug 27 '10 at 12:22
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Partial answer.

If $|f(z)|=1$ for all $|z|=1$ and $f$ is entire function, then $f:\mathbb D \to \mathbb D$, by the Maximum Modulus Theorem.

Using Schwarz-Lemma we can characterize, at least, the conformal ones.
( See Functions of one Complex Variable, Conway 2ed pag. 131 )

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This is meant as a comment to ifk's answer.

I don't see how you conclude from Schwarz's lemma that $g(z)=e^{iθ}z^k$, where k is the order of zero of g in 0. Moreover, the function $f$ you obtain is not entire :

$f=g \circ h^{-1}$, but $h^{-1}$ is not entire : it has a pole outside $\mathbb{D}$...

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No, ifk has $f(z)=g\circ h^{-1}(z)$. The function is entire if and only if a=0, so that $h^{−1}(z)=e^{i\omega}zh^{-1}(z)=e^{i\omega}z$ and $f(z)=e^{i(\theta+n\omega)}z^n$. –  George Lowther Aug 27 '10 at 0:28
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By maximum modulus principle we have that the zero inside the unit disc is at $0$.

Assume it has order $k$.

Hence $f(z)=z^k \cdot h(z)$ for some entire function $h$ with $h(0)\not = 0$.

Obviously we also have that $|h(z)|=1$ if $|z|=1$ so we can show that for all nonzero $z$, $h(z)={\overline{h(\frac{1}{\overline{z}})}}^{-1}$ so as $z$ goes to infinity, $h$ goes to a finite limit $\frac{1}{\overline{h(0)}}$ hence by Liouville's theorem, $h(z)=c$ for all $z$ with $|c|=1$ so $f(z)=cz^n$.

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Hi Gianluca , I have a simple question , I understand everything , $ f(z) = z^k h(z) $ with $h$ entire and $h(0) \ne 0$ , and $ |h(z)|=1 for |z|=1 $ . But I don't know how you concluded that $ h(z) = \frac{1}{\overline{h(\frac{1}{z})}}$ –  Daniel Oct 14 '12 at 16:16
    
Hi daniel, essentially the two functions agree on the unit circle, which has a limit point on C/{0} so they agree on all C/{0} by uniqueness theorem!! –  Moritzplatz Oct 24 '12 at 15:57
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@Gianluca how can you conclude at first that zero should be located at only 0? –  Detectives Nov 8 '12 at 8:24
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