The Boundary Harnack Principle for Nonlocal Elliptic Operators in Non-divergence Form

Abstract

We prove a boundary Harnack inequality for nonlocal elliptic operators $L$ in non-divergence form with bounded measurable coefficients. Namely, our main result establishes that if $L u_1=$ $L u_2=0$ in $\Omega \cap B_1, u_1=u_2=0$ in $B_1 \backslash \Omega$, and $u_1, u_2 \geq 0$ in $\mathbb{R}^n$, then $u_1$ and $u_2$ are comparable in $B_{1 / 2}$. The result applies to arbitrary open sets $\Omega$. When $\Omega$ is Lipschitz, we show that the quotient $u_1 / u_2$ is Hölder continuous up to the boundary in $B_{1 / 2}$. These results will be used in forthcoming works on obstacle-type problems for nonlocal operators.