Borderline Weak Type Estimates for Singular Integrals and Square Functions

TL;DR

This paper establishes near-endpoint weak type estimates for Calderón-Zygmund operators and square functions with weighted norms, extending classical results and providing sharp bounds involving logarithmic factors.

Contribution

It introduces new weak type bounds for Calderón-Zygmund operators and square functions with weights, improving upon prior results and including sharp logarithmic estimates.

Findings

Boundedness of Calderón-Zygmund operators from $L^1$ with complex Orlicz norms to weak-$L^1$.

Sharp $A_p$-weighted bounds for square functions involving logarithmic factors.

Extension of classical weighted inequalities to more delicate endpoint cases.

Abstract

For any Calder\'on-Zygmund operator $ T$, any weight $ w$, and $ \alpha >1$, the operator $ T$ is bounded as a map from $ L ^{1} (M _{ L \log\log L (\log\log\log L) ^{\alpha } } w )$ into weak-$L^1(w)$. The interest in questions of this type goes back to the beginnings of the weighted theory, with prior results, due to Coifman-Fefferman, P\'erez, and Hyt\"onen-P\'erez, on the $ L (\log L) ^{\epsilon }$ scale. Also, for square functions $ S f$, and weights $ w \in A_p$, the norm of $ S$ from $ L ^p (w)$ to weak-$L^p (w)$, $ 2\leq p < \infty $, is bounded by $ [w] _{A_p}^{1/2} (1+\log [w] _{A_ \infty }) ^{1/2} $, which is a sharp estimate.