Half-waves and spectral Riesz means on the 3-torus

Abstract

For a full rank lattice $\Lambda \subset \mathbb{R}^d$ and $\mathbf{A} \in \mathbb{R}^d$, consider $N_{d,0;\Lambda,\mathbf{A}}(\Sigma) = \# ([\Lambda+\mathbf{A}] \cap \Sigma \mathbb{B}^d) = \# \{\mathbf{k}\in \Lambda : |\mathbf{k}+\mathbf{A}| \leq \Sigma \}$. Consider the iterated integrals \[ N_{d,k+1;\Lambda,\mathbf{A}}(\Sigma) = \int_0^\Sigma N_{d,k;\Lambda,\mathbf{A}}(\sigma) \,\mathrm{d} \sigma, \] for $k\in \mathbb{N}$. After an elementary derivation via the Poisson summation formula of the sharp large-$\Sigma$ asymptotics of $N_{3,k;\Lambda,\mathbf{A}}(\Sigma)$ for $k\geq 2$ (these having an $O(\Sigma)$ error term), we discuss how they are encoded in the structure of the Fourier transform $\mathcal{F}N_{3;\Lambda,\mathbf{A}}(\tau)$. The analysis is related to H\"ormander's analysis of spectral Riesz means, as the iterated integrals above are weighted spectral Riesz means for the simplest magnetic Schr\"odinger operator on the flat $3$-torus. That the $N_{3,k;\Lambda,\mathbf{A}}(\Sigma)$ obey an asymptotic expansion to $O(\Sigma^2)$ is a special case of a general result holding for all magnetic Schr\"odinger operators on all manifolds, and the subleading polynomial corrections can be identified in terms of the Laurent series of the half-wave trace at $\tau=0$. The improvement to $O(\Sigma)$ for $k\geq 2$ follows from a bound on the growth rate of the half-wave trace at late times.