Pointwise 1/4-pinched 4-manifolds (Q1182223)

It is shown that a compact pointwise 1/4-pinched 4-manifold is either diffeomorphic to the sphere or real projective space or isometric to the complex projective plane. Note that 1/4-pinching does not imply that the curvature operator is non-negative. \textit{R. S. Hamilton} obtained a classification of compact 4-manifolds satisfying the latter condition [J. Differ. Geom. 24, 153-179 (1986; Zbl 0628.53042)].