Borderline Weak Type Estimates for Singular Integrals and Square Functions

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.