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JOURNALS // Zapiski Nauchnykh Seminarov POMI // Archive

Zap. Nauchn. Sem. POMI, 2006 Volume 337, Pages 233–237 (Mi znsl190)

This article is cited in 1 paper

Approximation by entire functions on subsets of a ray

O. V. Sil'vanovich, N. A. Shirokov

Saint-Petersburg State University

Abstract: Let $E\subset\mathbb R^+$ be a set consisting of finitely many intervals and a ray $[a,\infty)$, and let $H_\omega^r(E)$ be the set of functions defined on $E$ for which
$$ |f^{(r)}(x)-f^{(r)}(y)|\le c_f\omega(|x-y|), $$
where the continuity module $\omega(x)$ satisfies the condition
$$ \int_0^y\frac{\omega(x)}{x}dx+y\int_y^\infty\frac{\omega(x)}{x^2}dx\le C_0\omega(y), \quad y>0. $$
Let $C_\sigma^{(r,\omega)}$, $\sigma>0$, denote the class of entire functions $F$ of order 1/2 and of type $\sigma$ such that
$$ \sup_{z\in\mathbb C\setminus\mathbb R^+}\frac{|F(z)|e^{-\sigma|\operatorname{Im}\sqrt{z}|}}{1+|z|^r\omega(|z|)+\sigma^{-2r}\omega(\sigma^{-2})}<\infty. $$
In the paper, given a function $f\in H_\omega^r(E)$, we construct approximating functions $F$ in the class $C_\sigma^{(r,\omega)}$. Approximation by such functions on the set $E$ is analogous to approximation by polynomials on compacts. The analogy involves constructing a scale for measuring approximations and providing a constructive description of the class $H_\omega^r(E)$ in terms of the approximation rate, similar to that of polynomial approximation. Bibliography: 4 titles.

UDC: 511.44

Received: 28.08.2006


 English version:
Journal of Mathematical Sciences (New York), 2007, 143:3, 3149–3152

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