Small-scale mass estimates for Laplace eigenfunctions on compact $C^{2}$ manifolds with boundary

Abstract: Let $\Omega$ be an $n$-dimensional compact Riemannian manifold with $C^2$ boundary, and consider $L^2$-normalized eigenfunctions $ - \Delta \phi_{\lambda} = \lambda^2 \phi_\lambda$ with Dirichlet or Neumann boundary conditions . In this note, we extend well-known interior nonconcentration bounds up to the boundary. Specifically, in Theorem \ref{thm1} using purely stationary, local methods, we prove that for such $\Omega$, it follows that for {\em any} $x_0 \in \overline{\Omega}$ (including boundary points) and for all $\mu \geq h,$ \begin{equation} \label{nonconbdy} | \phi_\lambda |{B(x_0,\mu)\cap \Omega}^2 = O(\mu). \end{equation} In Theorem \ref{thm2} we extend a result of Sogge \cite{So} to manifolds with smooth boundary and show that \begin{equation} \label{SUPBD} | \phi\lambda |{L^\infty(\Omega)} \leq C \lambda^{\frac{n}{2}} \cdot \sup{x \in \Omega} | \phi_{\lambda} |{L^2( B(x,\lambda^{-1}) \cap \Omega )}. \end{equation} The sharp sup bounds $| \phi{\lambda} |_{L^\infty(\Omega)} = O(\lambda^{\frac{n-1}{2}})$ first proved by Grieser in \cite{Gr} are then an immediate consequence of Theorems \ref{thm1} and \ref{thm2}.