Mittag-Leffler functions in the Fourier space

Abstract: Let $\alpha \in (0,2)$ and let $\beta>0$. Fix $-\pi<\varphi\leq \pi$ such that $|\varphi|>\alpha \pi/2$. We determine the precise asymptotic behaviour of the Fourier transform of $E_{\alpha,\beta}(e^{\dot{\imath} \varphi} |\cdot|^{\sigma})$ whenever $\sigma>(n-1)/2$. Remarkably, this asymptotic behaviour turns out to be independent of $\alpha$ and $\beta$. This helps us determine the values of the Lebesgue exponent $p=p(\sigma)$, $\sigma>(n-1)/2$, for which $\mathcal{F} \left(E_{\alpha,\beta}(e^{\dot{\imath} \varphi} |\cdot|^{\sigma})\right)$ is in $L^{p}(\mathbb{R}^{n})$. These values cannot be obtained via the Hausdorff-Young inequality. This problem arises in the study of space-time fractional equations. Our approach provides an effective alternative to the asymptotic analysis of the Fox $H-$ functions recently applied to the case $\alpha \in (0,1)$, $\beta=\alpha, 1$, $\varphi=-\pi/2,\pi$. We rather rely on an appropriate integral representative that represents $E_{\alpha,\beta}$ continuously up to the origin, and we develop an extended asymptotic expansion for the Bessel function whose coefficients and remainder term are obtained explicitly.