Weyl remainders: an application of geodesic beams

TL;DR

This paper provides new quantitative estimates on Weyl Law remainders for Laplacian eigenfunctions on Riemannian manifolds, using geodesic beam techniques under dynamical assumptions on geodesic flow.

Contribution

It introduces novel bounds on Weyl Law remainders assuming small measure of near periodic geodesics, with applications to product manifolds and spectral projectors.

Findings

Improved remainder estimates for Weyl Law on manifolds with small measure near periodic geodesics.

Logarithmic gains in off-diagonal spectral projector asymptotics under non-looping assumptions.

Enhanced eigenvalue counting function estimates for product manifolds.

Abstract

We obtain new quantitative estimates on Weyl Law remainders under dynamical assumptions on the geodesic flow. On a smooth compact Riemannian manifold $(M,g)$ of dimension $n$, let $\Pi_\lambda$ denote the kernel of the spectral projector for the Laplacian, $\mathbb{1}_{[0,\lambda^2]}(-\Delta_g)$. Assuming only that the set of near periodic geodesics over $W\subset M$ has small measure, we prove that as $\lambda \to \infty$ $$ \int_{W} \Pi_\lambda(x,x)dx=(2\pi)^{-n}\text{vol}_{\mathbb{R}^n}(B)\text{vol}_g(W)\,\lambda^n+O\Big(\frac{\lambda^{n-1}}{\log \lambda }\Big),$$ where $B$ is the unit ball. One consequence of this result is that the improved remainder holds on all product manifolds, in particular giving improved estimates for the eigenvalue counting function in the product setup. Our results also include logarithmic gains on asymptotics for the off-diagonal spectral projector $\Pi_\lambda(x,y)$ under the assumption that the set of geodesics that pass near both $x$ and $y$ has small measure, and quantitative improvements for Kuznecov sums under non-looping type assumptions. The key technique used in our study of the spectral projector is that of geodesic beams.