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Fourier restriction in low fractal dimensions. (English) Zbl 1467.42018

Summary: Let \(S\subset\mathbb{R}^n\) be a smooth compact hypersurface with a strictly positive second fundamental form, \(E\) be the Fourier extension operator on \(S\), and \(X\) be a Lebesgue measurable subset of \(\mathbb{R}^n\). If \(X\) contains a ball of each radius, then the problem of determining the range of exponents \((p,q)\) for which the estimate \(\|Ef\|_{L^q(X)}\lesssim\|f\|_{L^p(S)}\) holds is equivalent to the restriction conjecture. In this paper, we study the estimate under the following assumption on the set \(X\): there is a number \(0 < \alpha\leq n\) such that \(|X\cap B_R|\lesssim R^{\alpha}\) for all balls \(B_R\) in \(\mathbb{R}^n\) of radius \(R\geq 1\). On the left-hand side of this estimate, we are integrating the function \(|Ef(x)|^q\) against the measure \(\chi_X{\mathrm{d}}x\). Our approach consists of replacing the characteristic function \(\chi_X\) of \(X\) by an appropriate weight function \(H\), and studying the resulting estimate in three different regimes: small values of \(\alpha\), intermediate values of \(\alpha\), and large values of \(\alpha\). In the first regime, we establish the estimate by using already available methods. In the second regime, we prove a weighted Hölder-type inequality that holds for general non-negative Lebesgue measurable functions on \(\mathbb{R}^n\) and combine it with the result from the first regime. In the third regime, we borrow a recent fractal Fourier restriction theorem of X. Du and R. Zhang [Ann. Math. (2) 189, No. 3, 837–861 (2019; Zbl 1433.42010)] and combine it with the result from the second regime. In the opposite direction, the results of this paper improve on the Du-Zhang theorem in the range \(0 < \alpha < n/2\).

MSC:

42B10 Fourier and Fourier-Stieltjes transforms and other transforms of Fourier type
42B20 Singular and oscillatory integrals (Calderón-Zygmund, etc.)
28A75 Length, area, volume, other geometric measure theory

Citations:

Zbl 1433.42010

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