圆不变脂肪丛与辛Fano 6-流形
Circle-invariant fat bundles and symplectic Fano 6-manifolds
摘要：
我们证明了一个紧的4-流形如果支持一个圆不变的胖SO（3）-丛，则它是S^4或CP^2-bar的同构。证明涉及研究所产生的哈密顿圈作用在相关的辛6-流形。将我们的结果应用到黎曼四维流形的扭量丛上，证明了S^4和CP^2-bar是唯一允许圆不变度量解某个曲率不等式的四维流形。这可以看作是向克林纳定理的一个类比，即只有S^4和CP^2允许正截面曲率的圆不变度量。
英文摘要：
We prove that a compact 4-manifold which supports a circle-invariant fat SO(3)-bundle is diffeomorphic to either S^4 or CP^2-bar. The proof involves studying the resulting Hamiltonian circle action on an associated symplectic 6-manifold. Applying our result to the twistor bundle of Riemannian 4-manifolds shows that S^4 and CP^2-bar are the only 4-manifolds admitting circle-invariant metrics solving a certain curvature inequality. This can be seen as an analogue of Hsiang-Kliener's theorem that only S^4 and CP^2 admit circle-invariant metrics of positive sectional curvature.
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