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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