Laplace's equation

In three dimensions, the problem is to find twice-differentiable real-valued functions φ of real variables x, y, and z such that

This is often written as

or

If the right-hand side is specified as a given function f(x, y, z), i.e.

then the equation is called Poisson's equation. These are the simplest examples of elliptic partial differential equations. The partial differential operator or (which may be defined in any number of dimensions) is called the Laplace operator or just the Laplacian.

The Dirichlet problem for Laplace's equation consists in finding a solution φ on some domain such that on the boundary of is equal to some given function. Since the Laplace operator appears in the heat equation, one physical interpretation of this problem is as follows: fix the temperature on the boundary of the domain and wait until the temperature in the interior doesn't change anymore; the temperature distribution in the interior will then be given by the solution to the corresponding Dirichlet problem.

The Neumann boundary conditions for Laplace's equation specify not the function itself on the boundary of , but its normal derivative. Physically, this corresponds to the construction of a potential for a vector field whose effect is known at the boundary of alone.

Solutions to Laplace's equation which are twice continuously differentiable are called harmonic functions; they are all analytic.

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