# Bonnet-Myers theorem

*This article describes a result related to the Ricci curvature of a Riemannian manifold*

*This result relates information on curvature to information on topology of a manifold*

*This article makes a prediction about the universal cover of a manifold based on given data at the level of a:*topological manifold

*This result is valid in all dimensions*

## Statement

If a Riemannian manifold has the property that there exists a positive constant that lower-bounds the Ricci curvature for all tangent lines at all points, then the manifold is compact with finite fundamental group. This is equivalent to saying that the universal cover of the manifold is compact.

Note that the condition on Ricci curvature is weaker than the corresponding condition on sectional curvature.

## Relation with other results

### Cartan-Hadamard theorem

`Further information: Cartan-Hadamard theorem`
The Cartan-Hadamard theorem talks of the analogous statement when the manifold has negative sectional curvature throughout. It says that under that assumption, the universal cover is diffeomorphic to real Euclidean space.

Together, the Cartan-Hadamard theorem and Bonnet-Myers theorem tell us that a manifold which has positive curvature bounded from below, cannot be diffeomorphic to a manifold which has negative sectional curvature throughout.