Extremizers for Fourier restriction on hyperboloids

Abstract: The $L^2 \to L^p$ adjoint Fourier restriction inequality on the $d$-dimensional hyperboloid $\mathbb{H}^d \subset \mathbb{R}^{d+1}$ holds provided $6 \leq p < \infty$, if $d=1$, and $2(d+2)/d \leq p\leq 2(d+1)/(d-1)$, if $d\geq2$. Quilodr\'{a}n recently found the values of the optimal constants in the endpoint cases $(d,p)\in{(2,4),(2,6),(3,4)}$ and showed that the inequality does not have extremizers in these cases. In this paper we answer two questions posed by Quilodr\'{a}n, namely: (i) we find the explicit value of the optimal constant in the endpoint case $(d,p) = (1,6)$ (the remaining endpoint for which $p$ is an even integer) and show that there are no extremizers in this case; and (ii) we establish the existence of extremizers in all non-endpoint cases in dimensions $d \in {1,2}$. This completes the qualitative description of this problem in low dimensions.