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Maximal theorems for the directional Hilbert transform on the plane
Authors:Michael T Lacey  Xiaochun Li
Institution:School of Mathematics, Georgia Institute of Technology, Atlanta, Georgia 30332 ; Department of Mathematics, University of California--Los Angeles, Los Angeles, California 90055-1555
Abstract:For a Schwartz function $ f$ on the plane and a non-zero $ v\in\mathbb{R}^2$ define the Hilbert transform of $ f$ in the direction $ v$ to be

$\displaystyle \operatorname H_vf(x)=$p.v.$\displaystyle \int_{\mathbb{R}} f(x-vy) \frac{dy}y. $

Let $ \zeta$ be a Schwartz function with frequency support in the annulus $ 1\le\vert\xi\vert\le2$, and $ {\boldsymbol \zeta}f=\zeta*f$. We prove that the maximal operator $ \sup_{\vert v\vert=1}\vert\operatorname H_v{\boldsymbol \zeta} f\vert$ maps $ L^2$ into weak $ L^2$, and $ L^p$ into $ L^p$ for $ p>2$. The $ L^2$ estimate is sharp. The method of proof is based upon techniques related to the pointwise convergence of Fourier series. Indeed, our main theorem implies this result on Fourier series.

Keywords:Hilbert transform  Fourier series  maximal function  pointwise convergence
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