Berger curvature decomposition, Weitzenb\"ock formula, and canonical metrics on four-manifolds

We first provide an alternative proof of the classical Weitzneb"ock formula for Einstein four-manifolds using Berger curvature decomposition, motivated by which we establish a unified framework for a Weitzenb"ock formula for a large class of canonical metrics on four-manifolds (or a Weitzenb"ock formula for “Einstein metrics” on four-dimensional smooth metric measure spaces). As applications, we classify Einstein four-manifolds of half two-nonnegative curvature operator, which in some sense provides a characterization of K"ahler-Einstein metrics with positive scalar curvature on four-manifolds, we also discuss four-manifolds of half two-nonnegative curvature operator and half harmonic Weyl curvature.