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Product Manifolds with Improved Spectral Cluster and Weyl Remainder Estimates

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 $λ\cdot S^{n-1}$ for $n\ge5$.