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Free interpolation in the spaces $ C^A_{r,\omega}$
N. A. Shirokov
Abstract:
Let the integer
$r\geqslant0$ and the modulus of continuity
$\omega(t)$ be fixed, and let
$C^A_{r,\omega}$ be the class of all functions continuous on the closed unit disk
$\overline D$, analytic on its interior
$D$, and having an
$\omega$-continuous
$r$th derivative on
$\overline D$.
Consider for each
$f\in C^A_{r,\omega}$ and each fixed
$\zeta\in\overline D$ the polynomial in
$z$
$$
P_{r,\zeta}(z;f)=\sum_{\nu=0}^r \dfrac{f^{(\nu)}(\zeta)}{\nu!}
$$
(the
$(r+1)$st partial sum of the Taylor series of
$f$ in a neighborhood of
$\zeta$). Then for any two points
$\zeta_1,\zeta_2\in\overline D$
\begin{equation}
\begin{gathered}
|(P_{r,\zeta_1}(z)-P_{r,\zeta_2}(z))^{(\nu)}|_{z=\zeta_1}\leqslant c_f|\zeta_1-\zeta_2|^{r-\nu}\omega(|\zeta_1-\zeta_2|),
\\
P_{\,\cdot\,,\,\cdot\,}(\,\cdot\,)=P_{\,\cdot\,,\,\cdot\,}(\,\cdot\,;f),\qquad 0\leqslant\nu\leqslant r.
\end{gathered}
\tag{1.1}
\end{equation}
Let
$E$ be a closed subset of
$\overline D$. This article contains a solution of the problem of free interpolation in
$C^A_{r,\omega}$, formulated as follows: find necessary and sufficient conditions on
$E$ such that for each collection
$\{P_\zeta\}_{\zeta\in E}$ of
$r$th-degree polynomials satisfying conditions of the type (1.1) for all
$\zeta_1,\zeta_2\in E$ there is a function
$f\in C^A_{r,\omega}$ with
$P_\zeta(\,\cdot\,)=P_{r,\zeta}(\,\cdot\,;f)$.
Bibliography: 13 titles.
UDC:
517.57
MSC: Primary
30E05; Secondary
26A15,
30C10,
46E15,
54C20 Received: 10.02.1981