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JOURNALS // Trudy Instituta Matematiki i Mekhaniki UrO RAN // Archive

Trudy Inst. Mat. i Mekh. UrO RAN, 2018 Volume 24, Number 4, Pages 104–109 (Mi timm1578)

This article is cited in 1 paper

Convergence of Trigonometric Fourier Series of Functions with a Constraint on the Fractality of Their Graphs

M. L. Gridnev

Institute of Mathematics and Mechanics, Ural Branch of the Russian Academy of Sciences, Ekaterinburg

Abstract: For a function $f$ continuous on a closed interval, its modulus of fractality $\nu(f,\varepsilon)$ is defined as the function that maps any $\varepsilon>0$ to the smallest number of squares of size $\varepsilon$ that cover the graph of $f$. The following condition for the uniform convergence of the Fourier series of $f$ is obtained in terms of the modulus of fractality and the modulus of continuity $\omega(f,\delta)$: if
$$ \omega (f,\pi/n) \ln\bigg(\frac{\nu(f,\pi/n)}{n}\bigg) \longrightarrow 0\ \ \ as \ n\longrightarrow+\infty, $$
then the Fourier series of $f$ converges uniformly. This condition refines the known Dini–Lipschitz test. In addition, for the growth order of the partial sums $S_n(f,x)$ of a continuous function $f$, we derive an estimate that is uniform in $x\in[0,2\pi]$:
$$ S_n(f,x)=o\bigg( \ln \bigg(\frac{\nu (f,\pi / n)}{n}\bigg)\bigg). $$
The optimality of this estimate is shown.

Keywords: trigonometric Fourier series, uniform convergence, fractal dimension.

UDC: 517.518.45

MSC: 42A20

Received: 31.08.2018
Revised: 28.10.2018
Accepted: 05.11.2018

DOI: 10.21538/0134-4889-2018-24-4-104-109


 English version:
Proceedings of the Steklov Institute of Mathematics (Supplementary issues), 2020, 308, suppl. 1, S106–S111

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© Steklov Math. Inst. of RAS, 2024