摘要

Let (M(4), g) be a four-dimensional complete noncompact Bach-flat Riemannian manifold with positive Yamabe constant. In this paper, we show that (M(4), g) has a constant curvature if it has a nonnegative constant scalar curvature and sufficiently small L(2)-norm of trace-free Riemannian curvature tensor. Moreover, we get a gap theorem for (M(4), g) with positive scalar curvature.