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Apr 15, 2017 at 14:02 comment added Yuhang Liu Yes. Taking product preserves positive Ricci curvature but not positive sectional curvature. $S^3\times S^3$ would also be an example. It seems somehow Fang could rule out those product manifolds(assuming SU(2) or SO(3) symmetry), leaving $S^2\times S^4$ as a possible exception. However I don't quite understand his arguments..
Apr 14, 2017 at 19:59 comment added Nick L I would add that this action preseves a metric with positive definite Ricci curvature (Since $S^{2} \times S^{2} \times S^{2}$ is a Fano 3-fold), which is still weaker than what you ask.
Apr 14, 2017 at 18:14 comment added Nick L Ahh this is more subtle than I thought. I do not know the answer to your question.
Apr 14, 2017 at 18:09 comment added Yuhang Liu Does S^2*S^2*S^2 admit positive sectional curvature? That's basically the Hopf conjecture.. Positivity of curvature puts strong restriction on topology.
Apr 14, 2017 at 15:56 history answered Nick L CC BY-SA 3.0