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Boundary distortions and change of module under extension of a doubly connected domain
A. Yu. Solynin
Abstract:
Let
$\mathcal{F}(p,r)$ denote the class of univalent analytic functions
$f(z)$ in the domain
$\mathcal{K}(\rho)=\{z: \rho<|z|<1\}$, satisfying
$|f(z)|=1$ for
$|z|=1$ and
$r<|f(z)|<1$ for
$z\in\mathcal{K}(\rho)$. Let
$f(z;\rho,r)$ map
$\mathcal{K}(\rho)$ onto the domain
$\mathcal{K}(r)\setminus[r,s]$ and let
$f(z;\rho,r)\in\mathcal{F}(\rho,r)$.
THEOREM 2. Let
$f(z)\in\mathcal{F}(\rho,r)$,
$f(z)\ne e^{i\alpha}f(z;\rho,r)$,
$\alpha\in\mathbb{R}$,
and
$\Phi(t)$ be a strictly convex monotone function of
$t>0$. Then
$$
\int_0^{2\pi}\Phi(|f'(e^{i\theta})|)d\theta<\int_0^{2\pi}\Phi(|f'(e^{i\theta};\rho,r)|)d\,\theta.
$$
The proof of this theorem is based on the Golusin–Komatu equation.
If
$E$ is a continuum in the disk
$U_R=\{z: |z|<R\}$, then
$M(R,E)$ denotes the conformal module of the doubly connected
component of
$U_R\setminus E$;
let $\varepsilon(m)=\{E: \overline{U}_r\subset E\subset U_1,\ M(1, E)=m^{-1}\}$.
PROBLEM. Find the maximum of
$M(R, E)$,
$R>1$, and the minimum
of cap
$E$ over all
$E$ in
$\varepsilon(m)$. This problem was posed by
V. V. Koževnikov in a lecture to the seminare on geometric function
theory at the Kuban university in 1980, and by D. Gaier (see [2]).
The solution of this problem is given by the following theorem.
THEOREM 3. Let
$E^*=\overline{U}_m\cup[m, s]$. If
$R>1; E, E^*\in\varepsilon(m)$
and
$E\ne e^{i\alpha}E^*$,
$\alpha\in\mathbb{R}$, then
$$
M(R, E)<M(R,E^*),\quad \mathrm{cap}\,E^*<\mathrm{cap}\,E.
$$
A similar statement is also proved for continue lying in the half-plane.
ADDENDUM. When the paper was ready for publication, the
author obtained a letter from R. Laugesen with the information
that he had also proved Theorem 3 by a different method based on
results of A. Baernstein. II on potential theory.
UDC:
517.54