Almost sharp local Bernstein estimates for Laplace eigenfunctions on compact Riemannian manifolds

Abstract

We study local growth properties of Laplace eigenfunctions on compact Riemannian manifolds. Following the paradigm introduced by Donnelly and Fefferman in the late 1980s, an eigenfunction is expected to behave locally like a polynomial of degree comparable to the square root of the eigenvalue. In this direction we establish almost sharp local $L^{p}$--Bernstein inequalities, $p\in[1,\infty]$, conjectured by Donnelly--Fefferman in 1990. We also derive analogous estimates for $A$-harmonic functions, with the square root of the eigenvalue replaced by the doubling index. Our argument refines the original Donnelly--Fefferman method based on $L^{2}$--Carleman estimates. At the $L^{2}$--level, we first prove a uniform bound for the doubling index on annuli of width comparable to the wavelength. This implies, with an arbitrarily small polynomial loss, the corresponding property at the $L^{p}$--level for all $p\in[1,\infty]$. The latter step relies on a bootstrap scheme combining elliptic regularity with a patching of local Carleman estimates on small balls.