Product Manifolds with Improved Spectral Cluster and Weyl Remainder Estimates

TL;DR

This paper demonstrates that improved spectral and eigenfunction estimates on a compact manifold Y lead to enhanced bounds on product manifolds X×Y, with applications to spheres and lattice point problems.

Contribution

It establishes that improvements in spectral estimates on Y extend to product manifolds X×Y, including optimal bounds for spheres with large q, partly confirming conjectures.

Findings

Improved eigenfunction estimates on Y imply better bounds on X×Y.

Enhanced Weyl remainder bounds on Y lead to spectral estimate improvements on X×Y.

Achieves optimal eigenfunction bounds for products of five or more spheres.

Abstract

We show that if $Y$ is a compact Riemannian manifold with improved $L^q$ eigenfunction estimates then, at least for large enough exponents, one always obtains improved $L^q$ bounds on the product manifold $X\times Y$ if $X$ is another compact manifold. Similarly, improved Weyl remainder term bounds on the spectral counting function of $Y$ lead to corresponding improvements on $X\times Y$. The latter results partly generalize recent ones of Iosevich and Wyman [14] involving products of spheres. Also, if $Y$ is a product of five or more spheres, we are able to obtain optimal $L^q(Y)$ and $L^q(X\times Y)$ eigenfunction and spectral cluster estimates for large $q$, which partly addresses a conjecture from [14] and is related to (and is partly based on) classical bounds for the number of integer lattice point on $\lambda \cdot S^{n-1}$ for $n\ge5$.