A short note on the dimension of the CC-geodesic Kakeya sets in the first Heisenberg group

Abstract

We study CC-geodesic Kakeya sets in the first Heisenberg group, namely Borel sets $E$ such that, for every unit-speed CC-geodesic segment of length \(1\) issuing from the identity, some left translate of the segment is contained in $E$. The natural analogue of the Kakeya conjecture would predict full Heisenberg Hausdorff dimension 4 for such sets. We show that this prediction fails: the sharp lower bound for their Heisenberg Hausdorff dimension is 3, and it remains sharp even among compact CC-geodesic Kakeya sets. By adjoining a Lebesgue-null set of full Heisenberg Hausdorff dimension, we obtain a CC-geodesic Kakeya set of full Heisenberg Hausdorff dimension 4 and zero Lebesgue measure. Finally, when one prescribes only geodesic segments with one fixed nonzero curvature parameter $\kappa$, rather than segments of all curvatures, the condition is weaker. For every $\kappa\in(0,2\pi]$, we construct a compact curvature-$\kappa$ Kakeya set of zero Lebesgue measure whose Euclidean and Heisenberg Hausdorff dimensions are both equal to $1$.