Unconditional convergence of the differences of Fejér kernels on $L^2(\mathbb{R})$
Izvestiâ vysših učebnyh zavedenij. Matematika, no. 8 (2024), pp. 27-33
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Let $K_n(x)$ denote the Fejér kernel given by $$K_n(x)=\sum_{j=-n}^n\left(1-\frac{|j|}{n+1}\right)e^{-ijx}$$ and let $\sigma_nf(x)=(K_n\ast f)(x)$, where as usual $f\ast g$ denotes the convolution of $f$ and $g$. Let the sequence $\{n_k\}$ be lacunary. Then the series $$\mathcal{G}f(x)=\sum_{k=1}^\infty \left(\sigma_{n_{k+1}}f(x)-\sigma_{n_k}f(x)\right)$$ converges unconditionally for all $f\in L^2(\mathbb{R})$. Let $(n_k)$ be a lacunary sequence, and $\{c_k\}_{k=1}^\infty \in \ell^\infty$. Define $$\mathcal{R}f(x)=\sum_{k=1}^\infty c_k\left(\sigma_{n_{k+1}}f(x)-\sigma_{n_k}f(x)\right).$$ Then there exists a constant $C>0$ such that $$\|\mathcal{R}f\|_2\leq C\|f\|_2$$ for all $f\in L^2(\mathbb{R})$, i.e., $\mathcal{R}f$ is of strong type $(2,2)$. As a special case it follows that $\mathcal{G}f$ also is of strong type $(2,2)$.
Keywords:
unconditional convergence
Mots-clés : Fejér kernel.
Mots-clés : Fejér kernel.
@article{IVM_2024_8_a2,
author = {S. Demir},
title = {Unconditional convergence of the differences of {Fej\'er} kernels on $L^2(\mathbb{R})$},
journal = {Izvesti\^a vys\v{s}ih u\v{c}ebnyh zavedenij. Matematika},
pages = {27--33},
year = {2024},
number = {8},
language = {ru},
url = {http://geodesic.mathdoc.fr/item/IVM_2024_8_a2/}
}
S. Demir. Unconditional convergence of the differences of Fejér kernels on $L^2(\mathbb{R})$. Izvestiâ vysših učebnyh zavedenij. Matematika, no. 8 (2024), pp. 27-33. http://geodesic.mathdoc.fr/item/IVM_2024_8_a2/