Fourier inequalities in variable Lebesgue spaces

We study the boundedness of the Fourier transform on variable Lebesgue spaces.

We obtain necessary conditions and, independently, sufficient conditions on the exponents $p(\cdot)$ and $q(\cdot)$ under which the inequality $$\|\hat{f}\|_{q(\cdot)}\leq C\|f\|_{p(\cdot)}$$ holds for some constant $C>0$ and all $f\in L^{p(\cdot)}(\mathbb{R})$.

As a byproduct, we improve some recent results of Saucedo and Tikhonov.

Moreover, when $p(x)\to 1$ and $q(x)\to +\infty$ as $|x|\to +\infty$, we characterize the exponents for which the Fourier transform is bounded.