In mathematics, the Neumann (or second-type) boundary condition is a type of boundary condition, named after Carl Neumann. When imposed on an ordinary or a partial differential equation, it specifies the values that the derivative of a solution is to take on the boundary of the domain.
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It is possible to describe the problem using other boundary conditions: a Dirichlet boundary condition specifies the values of the solution itself (as opposed to its derivative) on the boundary, whereas the Cauchy boundary condition, mixed boundary condition and Robin boundary condition are all different types of combinations of the Neumann and Dirichlet boundary conditions.
Ordinary differential equation
For an ordinary differential equation, for instance,
the Neumann boundary conditions on the interval
where
Partial differential equation
For a partial differential equation, for instance,
where
where
The normal derivative, which shows up on the left side, is defined as
where
It becomes clear that the boundary must be sufficiently smooth such that the normal derivative can exist, since for example at corner points of the boundary the normal vector is not well defined.
Applications
The following engineering applications involve the use of Neumann boundary conditions: