Sharp radial--angular mixed norm Fourier extension and global stability on spheres

Abstract

We study the sharp Fourier extension inequality from $L^2(\mathbb S^{d-1})$ to the radial--angular mixed space $L^q_{\mathrm{rad}}L^2_{\mathrm{ang}}$. Our work is motivated by and builds upon the foundational analysis of Carneiro, Oliveira e Silva and Sousa \cite{COS}, who initiated the sharp mixed-norm spherical restriction problem from the viewpoint of spherical-harmonic decomposition. Their reduction revealed that the extremal problem is governed by a competition among the individual harmonic modes, and their results established degree-zero extremizers in several important ranges of exponents, including even integer exponents and the endpoint case. The present paper addresses this question by establishing a complete modal ordering result. Building on their spherical-harmonic reduction, the sharp-constant problem is reduced to the weighted Bessel moments \[ I_{ν,k}(q) := \int_0^\infty |J_{ν+k}(r)|^q r^{2ν+1-νq}\,dr, \qquad ν:=\frac d2-1. \] For every integer $d\ge2$ and every finite admissible exponent $q>2d/(d-1)$, we prove \[ I_{ν,k}(q)<I_{ν,0}(q), \qquad k\ge1, \] and hence constants are the unique nonzero extremizers of the sharp mixed-norm extension inequality. In dimensions $d\ge3$ we prove the stronger strict ordering \[ I_{ν,0}(q)>I_{ν,1}(q)>I_{ν,2}(q)>\cdots. \] We resolve this difficulty by combining exact Hankel asymptotics with a finite remainder comparison. The modal ordering also yields the optimal quadratic global stability inequality. For every finite admissible $q$, \[ C_{d,q}^2\|f\|_2^2 -\|\widehat{fσ}\|_{L^q_{\mathrm{rad}}L^2_{\mathrm{ang}}}^2 \ge Γ_{d,q}\|f-P_0f\|_2^2, \] with the best possible coefficient. When $d\ge3$, this coefficient is explicitly \[ Γ_{d,q} =(2π)^d \left[ I_{ν,0}(q)^{2/q}-I_{ν,1}(q)^{2/q} \right]. \]