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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