The pointwise convergence of Fourier Series (II). Strong $L^1$ case for the lacunary Carleson operator

Abstract

We prove that the lacunary Carleson operator is bounded from $L \log L$ to $L^{1}$. This result is sharp. The proof is based on two newly introduced concepts: 1) the \emph{time-frequency regularization of a measurable set} and 2) the \emph{set-resolution of the time-frequency plane at $0-$frequency}. These two concepts will play the central role in providing a special tile decomposition adapted to the interaction between the \emph{structure} of the lacunary Carleson operator and the corresponding \emph{structure} of a fix measurable set. Another key insight of our paper is that it provides for the first time a simultaneous treatment of families of tiles with \emph{distinct} mass parameters. This should be regarded as a fundamental feature/difficulty of the problem of the pointwise convergence of Fourier Series near $L^1$, context in which, unlike the standard $L^p,\:p>1$ case, \emph{no decay} in the mass parameter is possible.