# User:OdedSchramm/sb2

## Ahlfors function

For each compact ${\displaystyle E\subset \mathbb {C} }$, there exists a unique extremal function, i.e. ${\displaystyle f\in {\mathcal {H}}^{\infty }(\mathbb {C} \setminus K)}$ such that ${\displaystyle \|f\|\leq 1}$, ${\displaystyle f(\infty )=0}$ and ${\displaystyle f'\,(\infty )=\gamma (K)}$. This function is called the Ahlfors function of K This can be proved by using a normal family argument involving Montel's theorem.

### Proof of existence for a continuum

There is a relatively simple proof of the existence of an Ahlfors function, based on the Riemann mapping theorem, if we assume additionally that K is connected.

If K is compact and connected, we can assume ${\displaystyle E\neq \varnothing }$ (otherwise ${\displaystyle {\mathcal {H}}^{\infty }(\mathbb {C} \setminus K)={\mathcal {H}}^{\infty }(\mathbb {C} )=\{{\text{constant functions}}\}}$ by Liouville's theorem and hence ${\displaystyle \gamma (K)=0}$). Then there exists a unique connected component U of ${\displaystyle {\hat {\mathbb {C} }}\setminus K}$ that contains ${\displaystyle \infty }$, where ${\displaystyle {\hat {\mathbb {C} }}=\mathbb {C} \cup \{\infty \}}$ is the Riemann sphere.

The claim is that U is simply connected. To see this, consider first a smooth simple closed curve ${\displaystyle \gamma }$ in ${\displaystyle U\cap \mathbb {C} }$ and let ${\displaystyle x_{0}}$ be some point in ${\displaystyle \gamma }$. By the Jordan curve theorem (actually, since ${\displaystyle \gamma }$ is smooth, one only needs easy versions of the Jordan curve theorem), ${\displaystyle \mathbb {C} \setminus \gamma }$ contains a connected component, say ${\displaystyle A}$ that is disjoint from ${\displaystyle K}$. Then ${\displaystyle A\subset U}$. Moreover, since ${\displaystyle \gamma }$ is smooth, the union ${\displaystyle \gamma \cup A}$ is homeomorphic to

The Riemann mapping theorem now yields a biholomorphism ${\displaystyle g\ :\ U\to B(0,1)}$ such that ${\displaystyle g(\infty )=0}$ and ${\displaystyle g'(\infty )>0}$. (Here, ${\displaystyle B(0,1)}$ denotes the unit disk in ${\displaystyle \mathbb {C} }$.) Defining ${\displaystyle g(z)=0}$ for each ${\displaystyle z\in \mathbb {C} \setminus (U\cup K)}$, this defines a holomorphic map ${\displaystyle g\ :\ \mathbb {C} \setminus K\to B(0,1)}$. In particular, ${\displaystyle g\in {\mathcal {H}}^{\infty }(\mathbb {C} \setminus E)}$, so that ${\displaystyle \gamma (E)\geq g'(\infty )}$.

To prove the reverse inequality, let ${\displaystyle f\in {\mathcal {H}}^{\infty }}$ with ${\displaystyle \|f\|_{\infty }\leq 1,\ f(\infty )=0}$ and put ${\displaystyle F:=f\circ g^{-1}}$. Then ${\displaystyle F\ :\ B(0,1)\to {\overline {B(0,1)}}}$ is analytic (since f and g are),

${\displaystyle F(0)=f(g^{-1}(0))=f(\infty )=0}$

and so we may apply the Schwarz lemma to F. Hence, ${\displaystyle F'(0)\leq 1}$. Thus,

${\displaystyle 1\geq F'(0)={\frac {f'(g^{-1}(0))}{g'(g^{-1}(0))}}={\frac {f'(\infty )}{g'(\infty )}}}$

which gives us ${\displaystyle f'(\infty )\leq g'(\infty )}$. Taking the supremum over all such f, we get ${\displaystyle \gamma (E)\leq g'(\infty )}$. This concludes the proof.

### Additional properties assuming finite connectivity

Let ${\displaystyle U:=\mathbb {C} \setminus E}$. If ${\displaystyle n\in \mathbb {N} }$ and E has n components, then the Ahlfors function is analytic across ${\displaystyle \partial U}$. Moreover, if ${\displaystyle \partial U}$ is smooth, then ${\displaystyle f(\partial U)=\{1\}}$.