This is often written as
where Δ is the Laplace operator.
Solutions of Laplace's equation are called harmonic functions.
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." Laplace's equation and Poisson's equation 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 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 of Laplace's equation are called harmonic functions; they are all analytic within the domain where the equation is satisfied. If any two functions are solutions to Laplace's equation (or any linear homogenous differential equation), their sum (or any linear combination) is also a solution. This property, called the principle of superposition, is very useful, e.g., solutions to complex problems can be constructed by summing simple solutions.
The Laplace equation in two independent variables has the form
then the necessary condition that f(z) be analytic is that the Cauchy-Riemann equations be satisfied:
It follows that
Therefore u satisfies the Laplace equation. A similar calculation shows that v also satisfies the Laplace equation.
Conversely, given a harmonic function, it is the real part of an analytic function, (at least locally). If a trial form is
then the Cauchy-Riemann equations will be satisfied if we set
This relation does not determine ψ, but only its increments:
The Laplace equation for φ implies that the integrability condition for ψ is satisfied:
and thus ψ may be defined by a line integral. The integrability condition and Stokes' theorem implies that the value of the line integral connecting two points is independent of the path. The resulting pair of solutions of the Laplace equation are called conjugate harmonic functions. This construction is only valid locally, or provided that the path does not loop around a singularity. For example, if r and θ are polar coordinates and
then a corresponding analytic function is
However, the angle θ is single-valued only in a region that does not enclose the origin.
The close connection between the Laplace equation and analytic functions implies that any solution of the Laplace equation has derivatives of all orders, and can be expanded in a power series, at least inside a circle that does not enclose a singularity. This is in sharp contrast to solutions of the wave equation, which generally have less regularity.
There is an intimate connection between power series and Fourier series. If we expand a function f in a power series inside a circle of radius R, this means that
with suitably defined coefficients whose real and imaginary parts are given by
which is a Fourier series for f.
Let the quantities u and v be the horizontal and vertical components of the velocity field of a steady incompressible, irrotational flow in two dimensions. The condition that the flow be incompressible is that
and the condition that the flow be irrotational is that
If we define the differential of a function ψ by
then the incompressibility condition is the integrability condition for this differential: the resulting function is called the stream function because it is constant along flow lines. The first derivatives of ψ are given by
and the irrotationality condition implies that ψ satisfies the Laplace equation. The harmonic function φ that is conjugate to ψ is called the velocity potential. The Cauchy-Riemann equations imply that
Thus every analytic function corresponds to a steady incompressible, irrotational fluid flow in the plane. The real part is the velocity potential, and the imaginary part is the stream function.
According to Maxwell's equations, an electric field (u,v) in two space dimensions that is independent of time satisfies
where ρ is the charge density. The first Maxwell equation is the integrability condition for the differential
so the electric potential φ may be constructed to satisfy
The second of Maxwell's equations then implies that
which is the Poisson equation.
It is important to note that the Laplace equation can be used in three-dimensional problems in electrostatics and fluid flow just as in two dimensions.
A fundamental solution of Laplace's equation satisfies
where the Dirac delta function denotes a unit source concentrated at the point No function has this property, but it can be thought of as a limit of functions whose integrals over space are unity, and whose support (the region where the function is non-zero) shrinks to a point (see weak solution). The definition of the fundamental solution thus implies that, if the Laplacian of u is integrated over any volume that encloses the source point, then
The Laplace equation is unchanged under a rotation of coordinates, and hence we can expect that a fundamental solution may be obtained among solutions that only depend upon the distance r from the source point. If we choose the volume to be a ball of radius a around the source point, then Gauss' divergence theorem implies that
It follows that
on a sphere of radius r that is centered around the source point, and hence
A similar argument shows that in two dimensions
A Green's function is a fundamental solution that also satisfies a suitable condition on the boundary S of a volume V. For instance, may satisfy
Now if u is any solution of the Poisson equation in V:
and u assumes the boundary values g on S, then we may apply Green's identity, (a consequence of the divergence theorem) which states that
The notations un and Gn denote normal derivatives on S. In view of the conditions satisfied by u and G, this result simplifies to
Thus the Green's function describes the influence at of the data f and g. For the case of the interior of a sphere of radius a, the Green's function may be obtained by means of a reflection (Sommerfeld, 1949): the source point P at distance ρ from the center of the sphere is reflected along its radial line to a point P' that is at a distance
Note that if P is inside the sphere, then P' will be outside the sphere. The Green's function is then given by
where R denotes the distance to the source point P and R' denotes the distance to the reflected point P. A consequence of this expression for the Green's function is the Poisson integral formula'. Let ρ, θ, and φ be spherical coordinates for the source point P. Here θ denotes the angle with the vertical axis, which is contrary to the usual American mathematical notation, but agrees with standard European and physical practice. Then the solution of the Laplace equation inside the sphere is given by
A simple consequence of this formula is that if u is a harmonic function, then the value of u at the center of the sphere is the mean value of its values on the sphere. This mean value property immediately implies that a non-constant harmonic function cannot assume its maximum value at an interior point.