A Kakeya maximal function estimate in four dimensions using planebrushes

Abstract

We obtain an improved Kakeya maximal function estimate in $\mathbb R^4$ using a new geometric argument called the planebrush. A planebrush is a higher dimensional analogue of Wolff’s hairbrush, which gives effective control on the size of Besicovitch sets when the lines through a typical point concentrate into a plane. When Besicovitch sets do not have this property, the existing trilinear estimates of Guth–Zahl can be used to bound the size of a Besicovitch set. In particular, we establish a maximal function estimate in $\mathbb R^4$ at dimension 3.059. As a consequence, every Besicovitch set in $\mathbb R^4$ must have Hausdorff dimension at least 3.059.