Dirichlet's principle

In mathematics, and particularly in potential theory, Dirichlet's principle is the assumption that the minimizer of a certain energy functional is a solution to Poisson's equation. Dirichlet's principle states that, if the function is the solution to Poisson's equation on a domain of with boundary condition on the boundary , then u can be obtained as the minimizer of the Dirichlet energy amongst all twice differentiable functions such that on (provided that there exists at least one function making the Dirichlet's integral finite). This concept is named after the German mathematician Peter Gustav Lejeune Dirichlet. The name "Dirichlet's principle" is due to Riemann, who applied it in the study of complex analytic functions. Riemann (and others such as Gauss and Dirichlet) knew that Dirichlet's integral is bounded below, which establishes the existence of an infimum; however, he took for granted the existence of a function that attains the minimum. Weierstrass published the first criticism of this assumption in 1870, giving an example of a functional that has a greatest lower bound which is not a minimum value. Weierstrass's example was the functional where is continuous on , continuously differentiable on , and subject to boundary conditions , where and are constants and . Weierstrass showed that , but no admissible function can make equal 0. This example did not disprove Dirichlet's principle per se, since the example integral is different from Dirichlet's integral. But it did undermine the reasoning that Riemann had used, and spurred interest in proving Dirichlet's principle as well as broader advancements in the calculus of variations and ultimately functional analysis. In 1900, Hilbert later justified Riemann's use of Dirichlet's principle by developing the direct method in the calculus of variations.