We consider second-order linear
elliptic operators of nondivergence type which are intrinsically defined on Riemannian
manifolds. Cabré proved a global Krylov-Safonov Harnack inequality under
the assumption that the sectional curvature is nonnegative. We improve
Cabré’s result and, as a consequence, we give another proof to the Harnack
inequality of Yau for positive harmonic functions on Riemannian manifolds with
nonnegative Ricci curvature using the nondivergence structure of the Laplace
operator.