On a Bernstein inequality for eigenfunctions

Abstract

Let $\varphi_{\lambda}$ be an eigenfunction of the Laplace-Beltrami operator on a smooth compact Riemannian manifold $(M,g)$, i.e., $\Delta_g \varphi_{\lambda} + \lambda \varphi_{\lambda}=0$. We show that $\varphi_{\lambda}$ satisfies a local Bernstein inequality, namely for any geodesic ball $B_g(x,r)$ in $M$ there holds: $\sup_{B_g(x,r)}|\nabla\varphi_{\lambda}|\leq C_{\delta}\max\left\{\frac{\sqrt{\lambda}\log^{2+\delta}\lambda}{r},\lambda\log^{2+\delta}\lambda\right\}\sup_{B_g(x,r)}|\varphi_{\lambda}|$. We also prove analogous inequalities for solutions of elliptic PDEs in terms of the frequency function.