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JOURNALS // Vestnik of Saint Petersburg University. Mathematics. Mechanics. Astronomy // Archive

Vestnik of Saint Petersburg University. Mathematics. Mechanics. Astronomy, 2020 Volume 7, Issue 3, Pages 481–489 (Mi vspua171)

This article is cited in 1 paper

MATHEMATICS

Approximation by entire functions on a countable set of continuums

O. V. Silvanovicha, N. A. Shirokovb

a St. Petersburg Mining University, 2, 21-ia liniia V. O., St.Petersburg, 199106, Russian Federation
b St. Petersburg State University, 7-9, Universitetskaya nab., St. Petersburg, 199034, Russian Federation

Abstract: We consider a problem of approximation by entire functions of exponential type of functions defined on a countable set $E$ of continuums $G_n$, $E = \bigcup_{n\in\mathbb{Z}} G_n$. We assume that all $G_n$ are pairwise disjoint and are situated near the real axis. We assume too that all $G_n$ are commensurable in a sense and have uniformly smooth boundaries. A function f is defined independantly on each $G_n$ and is bounded on $E$ and $f^{(r)}$ has a module of continuity $\omega$ which satisfies a condition $\int_0^x\omega(t)/t dt+x\int_x^\infty\omega(t)/t^2dt\leqslant c\omega(x)$. Then we construct an entire function $F_\sigma$ of exponential type $\leqslant\sigma$ such that we have the following estimate of approximation of the function $f$ by functions $F_\sigma$: $|f(z) - F_\sigma(z)| \leqslant c_f\sigma^{-r} \omega(\sigma^{-r}), z \in \mathbb{Z}, \sigma \leqslant 1$.

Keywords: Holder classes, approximation, entire functions of exponential type.

UDC: 517.537

MSC: 30E10

Received: 18.02.2019
Revised: 16.03.2020
Accepted: 19.03.2020

DOI: 10.21638/spbu01.2020.310


 English version:
Vestnik St. Petersburg University, Mathematics, 2020, 7:3, 329–335


© Steklov Math. Inst. of RAS, 2024