## 4-dim compact positively curved manifolds with a nontrivial Killing vector field.

Kleiner-Hsiang (JDG 1989) proved such a manifold is homeomorphic to $S^4$ or $CP^{2}$. an interesting corrollary is that $S^2 \times S^2$ does not admit positively curved metric with countinuous symmetry. They asked in their paper if it is diffeomorphic. I don't know much on this area, only noticed one subsequent work Searle-Yang. Seems that there are preprints on arXiv attempting to do the problem in full generality. Not sure if it was fully settled. Anyone knows the precise status of this problem?

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One preprint seems to have appeared in publication: worldscinet.com/ijm/22/2207/… – Agol Feb 4 2012 at 17:24
Thanks for pointing it out. I am just not sure if that paper is well acknowledged. – Bo Feb 9 2012 at 3:11