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JOURNALS // Matematicheskii Sbornik // Archive

Mat. Sb. (N.S.), 1976 Volume 100(142), Number 4(8), Pages 534–554 (Mi sm3016)

This article is cited in 20 papers

On uniform convergence of Fourier series

Z. A. Chanturiya


Abstract: Let $f(x)$ be a continuous $2\pi$-periodic function, $S_n(f, x)$ the $n$th partial sum of its Fourier series, $\omega(\delta,f)$ the modulus of continuity and $v(n,f)$ the modulus of variation of $f(x)$. In this paper the following theorems are proved.
Theorem 1. {\it For $f(x)\in C(0,2\pi)$ the estimate
$$ \|f(x)-S_n(f, x)\|_{C(0,2\pi)}\leqslant C\min_{1\leqslant m\leqslant[\frac{n-1}2]}\Biggl\{\omega\biggl(\frac1n,f\biggr)\sum_{k=1}^m\frac1k+\sum_{k=m+1}^{[\frac{n-1}2]}\frac{v(k,f)}{k^2}\Biggr\},\quad n\geqslant3, $$
holds, where $C$ is an absolute constant.}
From this theorem there follows an estimate of Lebesgue and an estimate of Oskolkov.
Theorem 2. {\it In order that all Fourier series of class $H^\omega\cap V[v(n)]$ converge uniformly it is necessary and sufficient that
$$ \lim_{n\to\infty}\min_{1\leqslant m\leqslant[\frac{n-1}2]}\Biggl\{\omega\biggl(\frac1n\biggr)\sum_{k=1}^m\frac1k+\sum_{k=m+1}^{[\frac{n-1}2]}\frac{v(k)}{k^2}\Biggr\}=0. $$
}
Bibliography: 20 titles.

UDC: 517.522.3

MSC: Primary 42A20; Secondary 26A15, 26A16, 26A45

Received: 08.08.1975


 English version:
Mathematics of the USSR-Sbornik, 1976, 29:4, 475–495

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© Steklov Math. Inst. of RAS, 2024