Zero-Plane Rigidity for Einstein Four-Manifolds
We prove that a closed Einstein four-manifold with positive Einstein constant, nonnegative sectional curvature, and a zero-curvature plane has universal Riemannian cover isometric to a product of two round two-spheres. Under the normalization $\mathop{\mathrm{Ric}}\nolimits =3g$, both spheres have radius $1/\sqrt3$. The proof extends coupled estimates for the two Weyl curvature blocks to the boundary of the sectional-curvature cone and determines their equality case.
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