Twisted Hilbert transforms vs Kakeya sets of directions

Abstract

Given a discrete group $\G$ and an orthogonal action $\gamma: \G \to O(n)$ we study $L_p$ convergence of Fourier integrals which are frequency supported on the semidirect product $\R^n \rtimes_\gamma \G$. Given a unit $u \in \R^n$ and $1 < p \neq 2 < \infty$, our main result shows that the twisted (directional) Hilbert transform $H_u \rtimes_\gamma id_\G$ is $L_p$-bounded iff the orbit $\mathcal{O}_\gamma(u)$ is finite. This is in sharp contrast with twisted Riesz transforms $R_u \rtimes_\gamma id_\G$, which are always bounded. Our result characterizes Fourier summability in $L_p$ for this class of groups. We also extend de Leeuw's compactification theorem to this setting and obtain stronger estimates for functions with "lacunary" frequency support.