====== Subharmonic function ======

A subharmonic function on a Riemannian manifold $M$ is a function $f : M \to \mathbb{R}$ with $\Delta f = \mathrm{d}^* \mathrm{d}f \le 0$.
Subharmonic functions satisfy the **maximum principle**: if a subharmonic function $f$ attains its maximum at an interior point of a domain of $M$, then $f$ is constant.