Boundary weak Harnack estimates and regularity for elliptic PDE in divergence form

Abstract

We obtain a global extension of the classical weak Harnack inequality which extends and quantifies the Hopf–Oleinik boundary-point lemma, for uniformly elliptic equations in divergence form. Among the consequences is a boundary gradient estimate, due to Krylov and well-studied for non-divergence form equations, but completely novel in the divergence framework. Another consequence is a new more general version of the Hopf–Oleinik lemma.