Unconditional convergence of the differences of Fejér kernels on $L^2(\mathbb{R})$

Abstract

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 ${\sigma }_{n}f(x)=(K_n\ast f)(x)$, 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 $$\Vert \mathcal{R}f{\Vert }_2\le C\Vert f{\Vert }_2$$ 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 $(2,2)$.

arXiv commentThis is a revised version of the present paper