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VARIABILITY REGIONS FOR THE nth DERIVATIVE OF BOUNDED ANALYTIC FUNCTIONS
Published online by Cambridge University Press: 14 February 2025
Abstract
Let $\mathcal {H}$ be the class of all analytic self-maps of the open unit disk
$\mathbb {D}$. Denote by
$H^n f(z)$ the nth-order hyperbolic derivative of
$f\in \mathcal H$ at
$z\in \mathbb {D}$. We develop a method allowing us to calculate higher-order hyperbolic derivatives in an expeditious manner. We also generalise certain classical results for variability regions of the nth derivative of bounded analytic functions. For
$z_0\in \mathbb {D}$ and
$\gamma = (\gamma _0, \gamma _1 , \ldots , \gamma _{n-1}) \in {\mathbb D}^{n}$, let
${\mathcal H} (\gamma ) = \{f \in {\mathcal H} : f (z_0) = \gamma _0,H^1f (z_0) = \gamma _1,\ldots ,H^{n-1}f (z_0) = \gamma _{n-1} \}$. We determine the variability region
$\{ f^{(n)}(z_0) : f \in {\mathcal H} (\gamma ) \}$ to prove a Schwarz–Pick lemma for the nth derivative. We apply this result to establish an nth-order Dieudonné lemma, which provides an explicit description of the variability region
$\{h^{(n)}(z_0): h\in \mathcal {H}, h(0)=0,h(z_0) =w_0, h'(z_0)=w_1,\ldots , h^{(n-1)}(z_0)=w_{n-1}\}$ for given
$z_0$,
$w_0$,
$w_1,\ldots ,w_{n-1}$. Moreover, we determine the form of all extremal functions.
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- Copyright
- © The Author(s), 2025. Published by Cambridge University Press on behalf of Australian Mathematical Publishing Association Inc.
Footnotes
This research was supported in part by the National Natural Science Foundation of China (Grant No. 12261059) and the China Scholarship Council (Grant No. 202308360150).