# Extensions of the vector-valued Hausdorff-Young inequalities

## TL;DR

This paper extends classical Fourier inequalities to vector-valued functions, establishing new bounds under geometric Banach space conditions and demonstrating sharpness through elementary examples.

## Contribution

It introduces vector-valued versions of several Fourier inequalities, including Pitt inequalities, under geometric conditions like Fourier type, and proves their sharpness with elementary examples.

## Key findings

- Vector-valued Hausdorff-Young inequalities established.
- Sharpness demonstrated via elementary examples on ll^p spaces.
- Connections with Rademacher type and cotype discussed.

## Abstract

In this paper we study the vector-valued analogues of several inequalities for the Fourier transform. In particular, we consider the inequalities of Hausdorff--Young, Hardy--Littlewood, Paley, Pitt, Bochkarev and Zygmund. The Pitt inequalities include the Hausdorff--Young and Hardy--Littlewood inequalities and state that the Fourier transform is bounded from $L^p(\mathbb{R}^d,|\cdot|^{\beta p})$ into $L^q(\mathbb{R}^d,|\cdot|^{-\gamma q})$ under certain condition on $p,q,\beta$ and $\gamma$. Vector-valued analogues are derived under geometric conditions on the underlying Banach space such as Fourier type and related geometric properties. Similar results are derived for $\mathbb{T}^d$ and $\mathbb{Z}^d$ by a transference argument. We prove sharpness of our results by providing elementary examples on $\ell^p$-spaces. Moreover, connections with Rademacher (co)type are discussed as well.