Dirichlet's principle states that, if the function is the solution to Poisson's equation
- 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.
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.
- Monna 1975, p. 33
- Monna 1975, p. 33–37,43–44
- Giaquinta and Hildebrand, p. 43–44
- Monna 1975, p. 55–56, citing Hilbert, David (1905), "Über das Dirichletsche Prinzip", Journal für die reine und angewandte Mathematik (in German), 129: 63–67
- Courant, R. (1950), Dirichlet's Principle, Conformal Mapping, and Minimal Surfaces. Appendix by M. Schiffer, Interscience
- Lawrence C. Evans (1998), Partial Differential Equations, American Mathematical Society, ISBN 978-0-8218-0772-9
- Giaquinta, Mariano; Hildebrandt, Stefan (1996), Calculus of Variations I, Springer
- A. F. Monna (1975), Dirichlet's principle: A mathematical comedy of errors and its influence on the development of analysis, Oosthoek, Scheltema & Holkema
- Weisstein, Eric W. "Dirichlet's Principle". MathWorld.