Soul theorem

Template:Short description In mathematics, the soul theorem is a theorem of Riemannian geometry that largely reduces the study of complete manifolds of non-negative sectional curvature to that of the compact case. Jeff Cheeger and Detlef Gromoll proved the theorem in 1972 by generalizing a 1969 result of Gromoll and Wolfgang Meyer. The related soul conjecture, formulated by Cheeger and Gromoll at that time, was proved twenty years later by Grigori Perelman.

Soul theorem

Cheeger and Gromoll's soul theorem states:Template:Sfnm

If Template:Math is a complete connected Riemannian manifold with nonnegative sectional curvature, then there exists a closed totally convex, totally geodesic embedded submanifold whose normal bundle is diffeomorphic to Template:Math.

Such a submanifold is called a soul of Template:Math. By the Gauss equation and total geodesicity, the induced Riemannian metric on the soul automatically has nonnegative sectional curvature. Gromoll and Meyer had earlier studied the case of positive sectional curvature, where they showed that a soul is given by a single point, and hence that Template:Mvar is diffeomorphic to Euclidean space.Template:Sfnm

Very simple examples, as below, show that the soul is not uniquely determined by Template:Math in general. However, Vladimir Sharafutdinov constructed a 1-Lipschitz retraction from Template:Mvar to any of its souls, thereby showing that any two souls are isometric. This mapping is known as the Sharafutdinov's retraction.Template:Sfnm

Cheeger and Gromoll also posed the converse question of whether there is a complete Riemannian metric of nonnegative sectional curvature on the total space of any vector bundle over a closed manifold of positive sectional curvature.Template:Sfnm The answer is now known to be negative, although the existence theory is not fully understood.Template:Sfnm

Examples.

• As can be directly seen from the definition, every compact manifold is its own soul. For this reason, the theorem is often stated only for non-compact manifolds.
• As a very simple example, take Template:Math to be Euclidean space Template:Math. The sectional curvature is Template:Math everywhere, and any point of Template:Math can serve as a soul of Template:Math.
• Now take the paraboloid Template:Math}, with the metric Template:Math being the ordinary Euclidean distance coming from the embedding of the paraboloid in Euclidean space Template:Math. Here the sectional curvature is positive everywhere, though not constant. The origin Template:Math is a soul of Template:Math. Not every point Template:Math of Template:Math is a soul of Template:Math, since there may be geodesic loops based at Template:Math, in which case <math>\{x\}</math> wouldn't be totally convex.Template:Sfnm
• One can also consider an infinite cylinder Template:Math}, again with the induced Euclidean metric. The sectional curvature is Template:Math everywhere. Any "horizontal" circle Template:Math} with fixed Template:Math is a soul of Template:Math. Non-horizontal cross sections of the cylinder are not souls since they are neither totally convex nor totally geodesic.Template:Sfnm

Soul conjecture

As mentioned above, Gromoll and Meyer proved that if Template:Mvar has positive sectional curvature then the soul is a point. Cheeger and Gromoll conjectured that this would hold even if Template:Mvar had nonnegative sectional curvature, with positivity only required of all sectional curvatures at a single point.Template:Sfnm This soul conjecture was proved by Grigori Perelman, who established the more powerful fact that Sharafutdinov's retraction is a Riemannian submersion, and even a submetry.Template:Sfnm

References

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