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Order estimates for Lebesgue constants of Fourier sums in Orlicz spaces
N. Yu. Antonova, A. N. Lukoyanovb a N.N. Krasovskii Institute of Mathematics and Mechanics, Ural Branch of the Russian Academy of Sciences, Ekaterinburg
b Ural Federal University named after the First President of Russia B. N. Yeltsin, Ekaterinburg
Abstract:
We consider the problem of order estimates for partial sums of trigonometric Fourier series as operators from Orlicz spaces $L^{\varphi}_{2\pi}$ to the space of $2\pi$-periodic continuous functions $C_{2\pi}$. It is established that an arbitrary function $\varphi$ generating an Orlicz class satisfies the estimate $$ ||S_n(f)||_{C_{2\pi}} \le C \varphi ^{-1} (n) \ln (n+1) ||f||_{L^{\varphi}_{2\pi}}, \tag{*} $$ where $f \in L^{\varphi}_{2\pi}$, $n \in \mathbb{N}$, $S_n(f)$ is the $n$th partial sum of the trigonometric Fourier series of $f$, and the constant $C>0$ is independent of $f$ and $n$. In addition, it is shown that if the function $\varphi$ satisfies the $\Delta_2$-condition, then the estimate can be improved. More exactly, $$ ||S_n(f)||_{C_{2\pi}} \le C \varphi ^{-1} (n) ||f||_{L^{\varphi}_{2\pi}}, \qquad f \in L^{\varphi}_{2\pi}, \, n \in \mathbb{N}, \, C=C(\varphi ). \tag {**} $$ Counterexamples are constructed, which show that if $\varphi$ satisfies the $\Delta_2$-condition, then estimate ($\ast \ast $) is unimprovable in order on the space $L^{\varphi}_{2\pi}$ and, if $\varphi$ satisfies the $\Delta^2$-condition, then estimate ($\ast $) is unimprovable in order on the space $ L^{\varphi}_{2\pi}$.
Keywords:
Fourier series, Orlicz space, Lebesgue constants.
Received: 28.07.2021 Revised: 25.10.2021 Accepted: 27.05.2021
Citation:
N. Yu. Antonov, A. N. Lukoyanov, “Order estimates for Lebesgue constants of Fourier sums in Orlicz spaces”, Trudy Inst. Mat. i Mekh. UrO RAN, 27, no. 4, 2021, 35–47
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https://www.mathnet.ru/eng/timm1861 https://www.mathnet.ru/eng/timm/v27/i4/p35
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Abstract page: | 197 | Full-text PDF : | 61 | References: | 41 | First page: | 12 |
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