A sharp $k$-plane Strichartz inequality for the Schrödinger equation

Abstract: We prove that $$ |X(|u|^2)|{L^3{t,\ell}}\leq C|f|_{L^2(\mathbb{R}^2)}^2, $$ where $u(x,t)$ is the solution to the linear time-dependent Schr\"odinger equation on $\mathbb{R}^2$ with initial datum $f$, and $X$ is the (spatial) X-ray transform on $\mathbb{R}^2$. In particular, we identify the best constant $C$ and show that a datum $f$ is an extremiser if and only if it is a gaussian. We also establish bounds of this type in higher dimensions $d$, where the X-ray transform is replaced by the $k$-plane transform for any $1\leq k\leq d-1$. In the process we obtain sharp $L^2(\mu)$ bounds on Fourier extension operators associated with certain high-dimensional spheres, involving measures $\mu$ supported on natural "co-$k$-planarity" sets.