Sharp Examples for Planar Quasiconformal Distortion of Hausdorff Measures and Removability

Abstract

In his celebrated paper on area distortion for quasiconformal mappings, Astala showed optimal area distortion bounds and dimension distortion estimates for planar quasiconformal mappings. He asked (Question 4.4) whether a finer result held, namely absolute continuity of Hausdorff measures under push-forward by quasiconformal mappings. This was proved in one particular case relevant for removability questions, in joint work of Astala, Clop, Mateu, Orobitg and the author ("Distortion of Hausdorff measures and improved Painlev'e removability for bounded quasiregular mappings", Duke Math J., to appear [ACMOU]) (Theorem 1.1), the other cases remaining open. A related question that we left open in [ACMOU] (Question 4.2) (which was asked by Astala to the author before [ACMOU] in an equivalent form in a personal communication) is whether BMO removability for K-quasiregular mappings and ($L^{\infty}$) removability for K-quasiregular mappings are indeed different problems. In this paper we give a series of examples answering in the positive Question 4.2 in [ACMOU], at the same time proving sharpness in two different senses of Theorem 1.1 in [ACMOU], and also giving examples that would yield sharpness in those two different senses as well for the absolute continuity of Hausdorff measures under push-forward by quasiconformal mappings, were it to be proved.