A. Ratto et M. Rigoli, GRADIENT BOUNDS AND LIOUVILLES TYPE THEOREMS FOR THE POISSON EQUATIONON COMPLETE RIEMANNIAN-MANIFOLDS, Tohoku Mathematical Journal, 47(4), 1995, pp. 509-519
We prove a gradient estimate and Liouville type theorems for the solut
ions of the Poisson equation on a complete manifold whose Ricci curvat
ure is suitably restricted.