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This article is cited in 5 scientific papers (total in 6 papers)
On the dependence of properties of functions on their degree of approximation by polynomials
E. P. Dolzhenko, E. A. Sevast'yanov
Abstract:
Let $f(x)$ be a bounded $2\pi$-periodic function with modulus of continuity $\omega(\delta,f)$; let $E_n(f)$ and $H_\alpha E_n(f)$ be the minimum deviations of $f$ from the trigonometric polynomials of order $\leqslant n$, in the uniform metric and the Hausdorff metric of order $\alpha$, respectively; let
$$
\sigma_n(f,\alpha)=H_\alpha E_0(f)+\dots+H_\alpha E_{n-1}(f).
$$
Then
\begin{gather*}
H_\alpha E_n(f)\leqslant E_n(f)\leqslant H_\alpha E_n(f)\exp\{(3+2\sqrt2\,)\alpha\sigma_n(f,\alpha)\},\\
\omega\left(\frac1n,f\right)\leqslant\frac{\exp\{(3+2\sqrt{2})\alpha{\sigma_n}(f,\alpha)\}-1}{n\alpha}.
\end{gather*}
If $H_\alpha E_n(f)\leqslant c/n\alpha$ as $n\to\infty$, then if $c<\pi$ the function $f$ is continuous almost everywhere; if $c<\pi/2$ it is continuous everywhere, and if $c<1$ we have $f\in\operatorname{Lip}\gamma(c)$, $\gamma(c)>0$.
Approximation by algebraic polynomials is also considered, and some corollaries are given.
Bibliography: 13 titles.
Received: 09.11.1976
Citation:
E. P. Dolzhenko, E. A. Sevast'yanov, “On the dependence of properties of functions on their degree of approximation by polynomials”, Izv. Akad. Nauk SSSR Ser. Mat., 42:2 (1978), 270–304; Math. USSR-Izv., 12:2 (1978), 255–288
Linking options:
https://www.mathnet.ru/eng/im1736https://doi.org/10.1070/IM1978v012n02ABEH001853 https://www.mathnet.ru/eng/im/v42/i2/p270
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Abstract page: | 321 | Russian version PDF: | 137 | English version PDF: | 9 | References: | 43 | First page: | 1 |
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