Myers's theorem

Myers's theorem, also known as the Bonnet–Myers theorem, is a celebrated, fundamental theorem in the mathematical field of Riemannian geometry. It was discovered by Sumner Byron Myers in 1941. It asserts the following:

Let $(M,g)$ be a complete Riemannian manifold of dimension $n$ whose Ricci curvature satisfies $\operatorname {Ric} (g)\geq (n-1)k$ for some positive real number $k.$ Then any two points of M can be joined by a geodesic segment of length at most ${\textstyle {\frac {\pi }{\sqrt {k}}}{\text{.}}}$

In the special case of surfaces, this result was proved by Ossian Bonnet in 1855. For a surface, the Gauss, sectional, and Ricci curvatures are all the same, but Bonnet's proof easily generalizes to higher dimensions if one assumes a positive lower bound on the sectional curvature. Myers' key contribution was therefore to show that a Ricci lower bound is all that is needed to reach the same conclusion.

Corollaries

The conclusion of the theorem says, in particular, that the diameter of $(M,g)$ is finite. The Hopf-Rinow theorem therefore implies that $M$ must be compact, as a closed (and hence compact) ball of radius $\pi /{\sqrt {k}}$ in any tangent space is carried onto all of $M$ by the exponential map.

As a very particular case, this shows that any complete and noncompact smooth Riemannian manifold which is Einstein must have nonpositive Einstein constant.

Consider the smooth universal covering map $\pi :N\to M.$ One may consider the Riemannian metric π^*g on $N.$ Since $\pi$ is a local diffeomorphism, Myers' theorem applies to the Riemannian manifold (N,π^*g) and hence $N$ is compact. This implies that the fundamental group of $M$is finite.

Cheng's diameter rigidity theorem

The conclusion of Myers' theorem says that for any $p,q\in M,$ one has d_g(p,q) ≤ π/√k. In 1975, Shiu-Yuen Cheng proved:

Let $(M,g)$ be a complete and smooth Riemannian manifold of dimension n. If k is a positive number with Ric^g ≥ (n-1)k, and if there exists p and q in M with d_g(p,q) = π/√k, then (M,g) is simply-connected and has constant sectional curvature k.

See also

• Gromov's compactness theorem (geometry) – On when a set of compact Riemannian manifolds of a given dimension is relatively compact

References

• Ambrose, W. A theorem of Myers. Duke Math. J. 24 (1957), 345–348.
• Cheng, Shiu Yuen (1975), "Eigenvalue comparison theorems and its geometric applications", Mathematische Zeitschrift, 143 (3): 289–297, doi:10.1007/BF01214381, ISSN 0025-5874, MR 0378001
• do Carmo, M. P. (1992), Riemannian Geometry, Boston, Mass.: Birkhäuser, ISBN 0-8176-3490-8
• Myers, S. B. (1941), "Riemannian manifolds with positive mean curvature", Duke Mathematical Journal, 8 (2): 401–404, doi:10.1215/S0012-7094-41-00832-3