Restricted convolution inequalities, multilinear operators and applications

TL;DR

This paper establishes new convolution inequalities and restriction theorems for functions on Euclidean spaces, leading to advances in multilinear analysis, including improved bounds for convolution operators, maximal functions, oscillatory integrals, and wave equation solutions.

Contribution

It introduces novel restricted convolution inequalities and multilinear restriction estimates, extending classical harmonic analysis results to multilinear and affine subspace contexts.

Findings

Proved new $L^p$-improving bounds for multilinear convolution measures.

Established an $m$-linear Stein spherical maximal theorem.

Derived estimates for multilinear oscillatory integrals and wave equation solutions.

Abstract

For $ 1\le k <n$, we prove that for functions $F,G$ on $ {\Bbb R}^{n}$, any $k$-dimensional affine subspace $H \subset {\Bbb R}^{n}$, and $p,q,r \ge 2$ with $\frac{1}{p}+\frac{1}{q}+\frac{1}{r}=1$, one has the estimate $$ {||(F*G)|_H||}_{L^{r}(H)} \leq {||F||}_{\Lambda^H_{2, p}({\Bbb R}^{n})} \cdot {||G||}_{\Lambda^H_{2, q}({\Bbb R}^{n})},$$ where the mixed norms on the right are defined by $$ {||F||}_{\Lambda^H_{2,p}({\Bbb R}^{n})}={(\int_{H^*} {(\int {|\hat{F}|}^2 dH_{\xi}^{\perp})}^{\frac{p}{2}} d\xi)}^{\frac{1}{p}},$$ with $dH_{\xi}^{\perp}$ the $(n-k)$-dimensional Lebesgue measure on the affine subspace $H_{\xi}^{\perp}:=\xi + H^\perp$. Dually, one obtains restriction theorems for the Fourier transform for affine subspaces. Applied to $F(x^{1},...,x^{m})=\prod_{j=1}^m f_j(x^{j})$ on $\R^{md}$, the diagonal $H_0={(x,...,x): x \in {\Bbb R}^d}$ and suitable kernels $G$, this implies new results for multilinear convolution operators, including $L^p$-improving bounds for measures, an $m$-linear variant of Stein's spherical maximal theorem, estimates for $m$-linear oscillatory integral operators, certain Sobolev trace inequalities, and bilinear estimates for solutions to the wave equation.