Weyl's lemma (Laplace equation)
In mathematics, Weyl's lemma, named after Hermann Weyl, states that every weak solution of Laplace's equation is a smooth solution. This contrasts with the wave equation, for example, which has weak solutions that are not smooth solutions. Weyl's lemma is a special case of elliptic or hypoelliptic regularity.
Statement of the lemma
Let be an open subset of -dimensional Euclidean space , and let denote the usual Laplace operator. Weyl's lemma states that if a locally integrable function is a weak solution of Laplace's equation, in the sense that
This result implies the interior regularity of harmonic functions in , but it does not say anything about their regularity on the boundary .
Idea of the proof
To prove Weyl's lemma, one convolves the function with an appropriate mollifier and shows that the mollification satisfies Laplace's equation, which implies that has the mean value property. Taking the limit as and using the properties of mollifiers, one finds that also has the mean value property, which implies that it is a smooth solution of Laplace's equation. Alternative proofs use the smoothness of the fundamental solution of the Laplacian or suitable a priori elliptic estimates.
Generalization to distributions
More generally, the same result holds for every distributional solution of Laplace's equation: If satisfies for every , then is a regular distribution associated with a smooth solution of Laplace's equation.
Connection with hypoellipticity
Weyl's lemma follows from more general results concerning the regularity properties of elliptic or hypoelliptic operators. A linear partial differential operator with smooth coefficients is hypoelliptic if the singular support of is equal to the singular support of for every distribution . The Laplace operator is hyperelliptic, so if , then the singular support of is empty since the singular support of is empty, meaning that . In fact, since the Laplacian is elliptic, a stronger result is true, and solutions of are real-analytic.
- Hermann Weyl, The method of orthogonal projections in potential theory, Duke Math. J., 7, 411-444 (1940). See Lemma 2, p. 415
- Bernard Dacorogna, Introduction to the Calculus of Variations, 2nd ed., Imperial College Press (2009), p. 148.
- Lars Gårding, Some Points of Analysis and their History, AMS (1997), p. 66.
- Lars Hörmander, The Analysis of Linear Partial Differential Operators I, 2nd ed., Springer-Verlag (1990), p.110