# Dirichlet energy

In mathematics, the Dirichlet energy is a measure of how variable a function is. More abstractly, it is a quadratic functional on the Sobolev space H1. The Dirichlet energy is intimately connected to Laplace's equation and is named after the German mathematician Peter Gustav Lejeune Dirichlet.

## Definition

Given an open set Ω ⊆ Rn and a function u : Ω → R the Dirichlet energy of the function u is the real number

${\displaystyle E[u]={\frac {1}{2}}\int _{\Omega }\|\nabla u(x)\|^{2}\,dx,}$

where u : Ω → Rn denotes the gradient vector field of the function u.

## Properties and applications

Since it is the integral of a non-negative quantity, the Dirichlet energy is itself non-negative, i.e. E[u] ≥ 0 for every function u.

Solving Laplace's equation ${\displaystyle -\Delta u(x)=0}$ for all ${\displaystyle x\in \Omega }$, subject to appropriate boundary conditions, is equivalent to solving the variational problem of finding a function u that satisfies the boundary conditions and has minimal Dirichlet energy.

Such a solution is called a harmonic function and such solutions are the topic of study in potential theory.