Spectral cluster estimates for Schrodinger operators of relativistic type

This paper is dedicated to L-p bounds on eigenfunctions of a Schrodinger-type operator (-Delta(g))(alpha/2) + V on closed Riemannian manifolds for critically singular potentials V. The operator (-Delta(g))(alpha/2) is defined spectrally in terms of the eigenfunctions of -Delta(g). We obtain also quasimodes and spectral clusters estimates. As an application, we derive Strichartz estimates for the fractional wave equation (partial derivative(2)(t)+(-Delta(g))(alpha/2)+ V) u = 0. The wave kernel techniques recently developed by Bourgain-Shao-Sogge-Yao [4] and Shao-Yao [27] play a key role in this paper. We construct a new reproducing operator with several local operators and some good error terms. Moreover, we shall prove that these local operators satisfy certain variable coefficient versions of the "uniform Sobolev estimates" by Kenig-Ruiz-Sogge [18]. These enable us to handle the critically singular potentials V and prove the quasimode estimates. (C) 2021 Elsevier Masson SAS. All rights reserved.