The Fourier transform on Rearrangement-Invariant Spaces

Abstract: We study inequalities of the form \begin{equation*} \rho ( \lvert \hat{f} \rvert) \leq C \sigma(f) < \infty, \end{equation*} with $f \in L_{1}(\mathbb{R}^n)$, the Lebesgue-integrable functions on $\mathbb{R}^n$ and \begin{equation*} \hat{f}(\xi) := \int_{\mathbb{R}^n} f(x) \, e^{- 2 \pi i \xi \cdot x} dx, \ \ \ \xi \in \mathbb{R}^n. \end{equation*} The functionals $\rho$ and $\sigma$ are so-called rearrangement-invariant (r.i.) norms on $M_{+}(\mathbb{R}^n)$, the nonnegative measurable functions on $\mathbb{R}^n$. Results first proved in the general context of r.i. spaces are then both specialized and expanded on in the special cases of Orlicz spaces and of Lorentz Gamma spaces.