Maximizers of the $L^2\to L^4$ Fourier extension inequality for cones in finite fields

Abstract

Sharp Fourier restriction theory and finite field extension theory have both been topics of interest in the last decades. Very recently, in \cite{GonzalezOliveira}, the research into the intersection of these two topics started. There it was established that, for the $(3,1)$-cone ${\Gamma }_{(3,1)}^3:=\{\mathbf{\eta }\in {\mathbb{F}}_q^4\setminus \{0\}:{\eta }_1^2+{\eta }_2^2+{\eta }_3^2={\eta }_4^2\},$ the Fourier extension map from $L^2\to L^4$ is maximized by constant functions when $q=3(\mathrm{mod}4)$. In this manuscript, we advance this line of inquiry by establishing sharp inequalities for the $L^2\to L^4$ extension inequalities applicable for all remaining cones ${\Gamma }^3\subset {\mathbb{F}}_q^4$. These cones include the $(2,2)$-cone ${\Gamma }_{(2,2)}^3:=\{\mathbf{\eta }\in {\mathbb{F}}_q^4\setminus \{0\}:{\eta }_1^2+{\eta }_2^2={\eta }_3^2+{\eta }_4^2\}$ for general $q=p^n$ and the $(3,1)$-cone when $q=1(\mathrm{mod}4)$. Moreover, we classify all the extremizers in each case. We note that the analogous problem for the $(2,2)$-cone in the euclidean setting remains open.

17 pages. v2: small typos corrected, acknowledgements added