# Mixed boundary condition

Green: Neumann boundary condition; purple: Dirichlet boundary condition.

In mathematics, a mixed boundary condition for a partial differential equation defines a boundary value problem in which the solution of the given equation is required to satisfy different boundary conditions on disjoint parts of the boundary of the domain where the condition is stated. Precisely, in a mixed boundary value problem, the solution is required to satisfy a Dirichlet or a Neumann boundary condition in a mutually exclusive way on disjoint parts of the boundary.

For example, given a solution u to a partial differential equation on a domain Ω with boundary ∂Ω, it is said to satisfy a mixed boundary condition if, consisting ∂Ω of two disjoint parts, Γ
1
and Γ
2
, such that ∂Ω = Γ
1
∪ Γ
2
, u verifies the following equations:

$u_{\big| \Gamma_1} = u_0$
$\left. \frac{\partial u}{\partial n}\right|_{\Gamma_2} = g$

where u
0
and g are given functions defined on those portions of the boundary.1

The mixed boundary condition differs from the Robin boundary condition in that this last one requires a linear combination, possibly with pointwise variable coefficients, of the Dirichlet and the Neumann bondary value conditions to be satisfied on the whole boundary of a given domain.

## Historical note

M. Wirtinger, dans une conversation privée, a attiré mon attention sur le probleme suivant: déterminer une fonction u vérifiant l'équation de Laplace dans un certain domaine (D) étant donné, sur une partie (S) de la frontière, les valeurs périphériques de la fonction demandée et, sur le reste (S′) de la frontière du domaine considéré, celles de la dérivée suivant la normale. Je me propose de faire connaitre une solution très générale de cet intéressant problème.2

Stanisław Zaremba(Zaremba 1910, §1, p. 313).

The first boundary value problem satisfying a mixed boundary condition was solved by Stanisław Zaremba for the Laplace equation: according to himself, he was attracted to the study of this problem by Wilhelm Wirtinger.3

## Notes

1. ^ Obviously, it is not at all necessary to require u
0
and g being functions: they can be distributions or any other kind of generalized functions.
2. ^ (English translation) "Mr. Wirtinger, during a private conversation, has attracted my attention on the following problem: to determine one function u solving Laplace s equation on a certain domain (D) being given, on a part (S) of its boundary, the peripheral values of the sought function and, on the remaining part (S′) of the considered domain, the ones of its derivative along the normal. I aim to make known a very general solution of this interesting problem."
3. ^ See (Zaremba 1910, §1, p. 313).

## References

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