Almost non-negatively curved 4-manifolds with torus symmetry

Abstract

We prove that if a closed, smooth, simply-connected 4-manifold with a circle action admits an almost non-negatively curved sequence of invariant Riemannian metrics, then it also admits a non-negatively curved Riemannian metric invariant with respect to the same action. The same is shown for torus actions of higher rank, giving a classification of closed, smooth, simply-connected 4-manifolds of almost non-negative curvature under the assumption of torus symmetry.

15 pages. As suggested by a referee for Proc. Amer. Math. Soc., the result is expanded in this version to include all torus actions, with the title changing accordingly