Hardy-Littlewood-type theorems for Fourier transforms in $\R^d$

Abstract: We obtain Fourier inequalities in the weighted $L_p$ spaces for any $1<p<\infty$ involving the Hardy-Ces`aro and Hardy-Bellman operators. We extend these results to product Hardy spaces for $p\le 1$. Moreover, boundedness of the Hardy-Ces`aro and Hardy-Bellman operators in various spaces (Lebesgue, Hardy, BMO) is discussed. One of our main tools is an appropriate version of the Hardy-Littlewood-Paley inequality $ |\widehat{f}|{L{p',q}} \lesssim \left|f\right|{L{p,q}}$.