Let $(M^n,g)$ be a [complete Riemannian manifold](/page/Complete%20Riemannian%20Manifold) of dimension $n$ with nonnegative [Ricci curvature](/page/Ricci%20Curvature), and let $\rho$ be a [ray](/page/Ray). Then the [Busemann function](/page/Busemann%20Function) $b_\rho$ is [subharmonic](/page/Subharmonic%20Function) in the [distributional sense](/page/Distribution).