In mathematics, the Bishop–Gromov inequality is a comparison theorem in Riemannian geometry, named after Richard L. Bishop and Mikhail Gromov. It is closely related to Myers' theorem, and is the key point in the proof of Gromov's compactness theorem.
Let be a complete n-dimensional Riemannian manifold whose Ricci curvature satisfies the lower bound
for a constant . Let be the complete n-dimensional simply connected space of constant sectional curvature (and hence of constant Ricci curvature ); thus is the n-sphere of radius if K > 0, or n-dimensional Euclidean space if , or an appropriately rescaled version of n-dimensional hyperbolic space if . Denote by B(p, r) the ball of radius r around a point p, defined with respect to the Riemannian distance function.
Then, for any and , the function
is non-increasing on (0, ∞).
As r goes to zero, the ratio approaches one, so together with the monotonicity this implies that