Pointwise Riesz potential bounds for rough singular integrals

Abstract

We give a complete characterization of the kernels $Ω\in \Lup^1(\Sp)$ for which the rough singular integral $\T_Ω$ and the associated maximal operator $\M_Ω$ satisfy pointwise bounds by $\Int_1(|\nabla \cdot|)$ and $\M_1(|\nabla \cdot|)$, respectively, where $\Int_1$ is the Riesz potential and $\M_1$ is the fractional maximal operator. These bounds hold if and only if $\Omega$ belongs to the homogeneous dual Sobolev space $\dot \W^{-1,\infty}(\Sp)$. Equivalently, $Ω=\text{div}_\Sp \mathbf{V}$ for some bounded vector field $\mathbf{V}$ on the sphere, or $\left|{\int }_E\Omega \right|\le C{\mathcal{H}}^{d-2}(\partial E)$ for every open set $E\subseteq\Sp$ with smooth boundary. On the Lebesgue scale, we show that for $d\ge 3$ the condition $Ω\in\Lup^{d-1,\infty}(\Sp)$, together with mean zero, suffices, and that the exponent $d-1$ is sharp. Finally, in dimension $d=2$, we use complex-analytic methods to prove a new pointwise bound in terms of the complex derivative $\bar{\partial }f$ that is valid for every mean-zero $Ω\in\Lup^1(\mathbb S^1)$. As a consequence, we obtain new Sobolev bounds for rough singular integrals in the plane.