On the eigenvalues of the Laplacian on fibred manifolds (2408.07880v1)

Abstract: We prove various comparison theorems of the $i$-th eigenvalue $\lambda_i$ of the Laplacian on fibred Riemannian manifolds by using fiberwise spherical and Euclidean (or hyperbolic) symmetrization. In particular we generalize the Lichnerowicz inequality and the Faber-Krahn inequality to fiber bundles, and prove a counterpart to Cheng's $\lambda_1$ comparison theorem under a lower Ricci curvature bound. By applying these, it is shown that $\lambda_1,\cdots,\lambda_k$ of a fiber bundle given by a Riemannian submersion with totally geodesic fibers of sufficiently positive Ricci curvature are respectively equal to $\lambda_1,\cdots,\lambda_k$ of its base, and $\lambda_1$ of a (possibly singular) fibration with Euclidean subsets as fibers is no less than $\lambda_1$ of the disk bundle obtained by replacing each fiber with a Euclidean disk of the same dimension and volume.