2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/170545Consider positive solutions to second order elliptic equations with measurable coefficients in a bounded domain, which vanish on a portion of the boundary. We give simple necessary and sufficient geometric conditions on the domain, which guarantee the Hopf-Oleinik type estimates and the boundary Lipschitz estimates for solutions. These conditions are sharp even for harmonic functions.20 pages, 23 referencesAnalysis of PDEs35J15, 35J67 (Primary). 35B05 (Secondary)text