We refine a theorem due to Gursky (2000). As applications, we give some rigidity
theorems on four-manifolds with positive Yamabe constant. We recover some
of Gursky’s results (1998, 2000). We prove some classification theorems of
four-manifolds according to some conformal invariants, which reprove and
generalize the conformally invariant sphere theorem of Chang, Gursky and Yang
(2003).
Keywords
four-manifold, Einstein manifold, harmonic curvature,
harmonic Weyl tensor, Yamabe constant