Dirichlet energy on a Riemannian manifold (source code)

= Dirichlet energy on a Riemannian manifold
{title2=$E(u)$}
{wiki=Dirichlet_energy}

For a function $u$ on a compact Riemannian manifold, its Dirichlet energy is $\frac12\int_M|\nabla u|_g^2d\operatorname{vol}_g$. Adding a linear source term $-\int_Mfu\,d\operatorname{vol}_g$ gives Euler-Lagrange equation $-\Delta_gu=f$.