In this paper, we prove a gap result for a locally conformally flat complete
non-compact Riemannian manifold with bounded non-negative Ricci curvature and a
scalar curvature average condition. We show that if it has positive Green
function, then it is flat. This result is proved by setting up new global
Yamabe flow. Other extensions related to bounded positive solutions to a
schrodinger equation are also discussed.Comment: Accepted version in Journal of Differential Equatrio