Almost everywhere summability of Fourier series with indicating the set of convergence

Abstract

The following problem is studied in this paper: Which multipliers $\{\lambda_{k, n}\}$ ensure the convergence, as $n\to \infty$, of the linear means of the Fourier series of functions $f\in L_1[-\pi, \pi]$ $$ \sum_{k=-\infty}^\infty \lambda_{k, n}\hat{f}_k e^{ikx}, $$ where $\widehat{f}_k$ is the $k$-th Fourier coefficient, at a point at which the derivative of the function $\int_0^x f$ exists. A criterion for the convergence of the $(C, 1)$-means ($\lambda_{k, n}=(1-\frac {|k|}{n+1})_+$) is found, while in the general case $\lambda_{k, n}=\phi(\frac {k}{n+1})$ a sufficient condition is derived for the convergence at all such points (that is, almost everywhere). The answer is given in terms of the belonging of $\phi(x)$ and $x\phi'(x)$ to the Wiener algebra of absolutely convergent Fourier integrals. The obtained results are supplemented by some examples.