On the problem of existence in principal value of a Calderón–Zygmund operator on a space of non‐homogeneous type

Abstract

In this paper, we study the relationship between two fundamental regularity properties of an s-dimensional Calderón–Zygmund operator (CZO) acting on a Borel measure μ in R d , with s ∈ ( 0 , d ) . In the classical case when s = d and μ is equal to the Lebesgue measure, Calderón and Zygmund showed that if a CZO is bounded in L 2 , then the principal value integral exists almost everywhere. However, there are by now several examples showing that this implication may fail for lower dimensional kernels and measures, even when the CZO has a homogeneous kernel consisting of spherical harmonics. We introduce sharp geometric conditions on μ, in terms of certain scaled transportation distances, which ensure that an extension of the Calderón–Zygmund theorem holds. These conditions are necessary and sufficient in the cases of the Riesz transform and the Huovinen transform. Our techniques build upon prior work by Mattila and Verdera, and incorporate the machinery of symmetric measures, introduced to the area by Mattila.