Sharp Spectral Projection Estimates for the Torus at $p_c=\frac{2(n+1)}{n-1}$

Abstract

We prove sharp spectral projection estimates for tori in all dimensions at the exponent $p_c=\frac{2(n+1)}{n-1}$ for shrinking windows of width $1$ down to windows of length $\lambda^{-1+\kappa}$ for fixed $\kappa>0$. This improves and slightly generalizes the work of Blair-Huang-Sogge who proved sharp results for windows of width $\lambda^{-\frac{1}{n+3}}$, and the work of Hickman, Germain-Myerson, and Demeter-Germain who proved results for windows of all widths but incurred a sub-polynomial loss. Our work uses the approaches of these two groups of authors, combining the bilinear decomposition and microlocal techniques of Blair-Huang-Sogge with the decoupling theory and explicit lattice point lemmas used by Hickman, Germain-Myerson, and Demeter-Germain to remove these losses.