$L^4$ norm of spectral projectors on polynomially small frequency intervals for $S^1$-symmetric surfaces

Abstract

For $(M,g)$ a compact Riemannian surface with Laplace-Beltrami operator $\Delta$, and for $\lambda ,\delta \ge 0$, let $P_{\lambda ,\delta }$ be the spectral projector on the frequency interval $[\lambda -\delta ,\lambda +\delta ]$ associated to $\sqrt{-\Delta }$. For the Euclidean disk, away from its boundary, we improve the upper bound on the $L^2\to L^4$ norm of $P_{\lambda ,\delta }$ in the regime where the bandwidth $\delta$ is polynomially small compared to the target frequency $\lambda$. Decomposing on the explicit joint eigenbasis of $\left(\sqrt{-\Delta },\frac{1}{i}\frac{\partial }{\partial \theta }\right)$ given in terms of Bessel eigenfunctions, which are well-approximated by oscillatory functions outside of their caustic set, we reduce the analysis to a number of precise quantitative estimates of nonstationary phase oscillatory integrals. We strongly use convexity phenomenon both for these estimates, and then for the summation of the contribution of all eigenfunctions through a new arithmetic estimate. The method extends to other $S^1$-symmetric surfaces satisfying similar conditions on the induced completely integrable structure.