Spectral rigidity for spherically symmetric manifolds with boundary

We prove a trace formula for three-dimensional spherically symmetric Riemannian manifolds with boundary which satisfy the Herglotz condition: Under a "clean intersection hypothesis" and assuming an injectivity hypothesis associated to the length spectrum, the wave trace is singular at the lengths of periodic broken rays. In particular, the Neumann spectrum of the Laplace-Beltrami operator uniquely determines the length spectrum. The trace formula also applies for the toroidal modes of the free oscillations in the earth. Under this hypothesis and the Herglotz condition, we then prove that the length spectrum is rigid: Deformations preserving the length spectrum and spherical symmetry are necessarily trivial in any dimension, provided the Herglotz condition and a geometrical condition are satisfied. Combining the two results shows that the Neumann spectrum of the Laplace-Beltrami operator is rigid in this class of manifolds with boundary. (c) 2021 Elsevier Masson SAS. All rights reserved.