This important paper shows how to establish weak type estimates for singular integral operators (and related operators) when the kernel is not smooth enough to satisfy a Hörmander condition:
for some in . Roughly speaking, this estimate is replaced by
where is the kernel of , being a suitable approximate identity, with kernel dominated by , where is a bounded positive decreasing function vanishing at a prescribed polynomial rate at infinity. A Calderón-Zygmund decomposition is used, and a tricky argument applies to estimate the “bad part”. It turns out to be possible to prove the second estimate in a range of contexts. The applications include estimates for singular integrals on domains with irregular boundaries, maximal functions associated to singular integrals, Riesz transforms and functions of operators which generate semigroups with kernels which satisfy estimates like above.