Harmonic function

In mathematics, mathematical physics and the theory of stochastic processes, a harmonic function is a twice continuously differentiable function where is an open subset of that satisfies Laplace's equation, that is,
everywhere on. This is usually written as
or

Etymology of the term "harmonic"

The descriptor "harmonic" in the name "harmonic function" originates from a point on a taut string which is undergoing harmonic motion. The solution to the differential equation for this type of motion can be written in terms of sines and cosines, functions which are thus referred to as "harmonics." Fourier analysis involves expanding functions on the unit circle in terms of a series of these harmonics. Considering higher dimensional analogues of the harmonics on the unit n-sphere, one arrives at the spherical harmonics. These functions satisfy Laplace's equation and, over time, "harmonic" was used to refer to all functions satisfying Laplace's equation.

Examples

Examples of harmonic functions of two variables are:

The real or imaginary part of any holomorphic function. In fact, all harmonic functions defined on the plane are of this form.
The function this is a special case of the example above, as and is a holomorphic function. The second derivative with respect to x is while the second derivative with respect to y is
The function defined on This can describe the electric potential due to a line charge or the gravity potential due to a long cylindrical mass.

Examples of harmonic functions of three variables are given in the table below with
Harmonic functions that arise in physics are determined by their singularities and boundary conditions. On regions without boundaries, adding the real or imaginary part of any entire function will produce a harmonic function with the same singularity, so in this case the harmonic function is not determined by its singularities; however, we can make the solution unique in physical situations by requiring that the solution approaches 0 as r approaches infinity. In this case, uniqueness follows by Liouville's theorem.
The singular points of the harmonic functions above are expressed as "charges" and "charge densities" using the terminology of electrostatics, and so the corresponding harmonic function will be proportional to the electrostatic potential due to these charge distributions. Each function above will yield another harmonic function when multiplied by a constant, rotated, and/or has a constant added. The inversion of each function will yield another harmonic function which has singularities which are the images of the original singularities in a spherical "mirror". Also, the sum of any two harmonic functions will yield another harmonic function.
Finally, examples of harmonic functions of variables are:

The constant, linear and affine functions on all of
The function on for.
Properties

The set of harmonic functions on a given open set can be seen as the kernel of the Laplace operator and is therefore a vector space over linear combinations of harmonic functions are again harmonic.
If is a harmonic function on, then all partial derivatives of are also harmonic functions on. The Laplace operator and the partial derivative operator will commute on this class of functions.
In several ways, the harmonic functions are real analogues to holomorphic functions. All harmonic functions are analytic, that is, they can be locally expressed as power series. This is a general fact about elliptic operators, of which the Laplacian is a major example.
The uniform limit of a convergent sequence of harmonic functions is still harmonic. This is true because every continuous function satisfying the mean value property is harmonic. Consider the sequence on defined by this sequence is harmonic and converges uniformly to the zero function; however note that the partial derivatives are not uniformly convergent to the zero function. This example shows the importance of relying on the mean value property and continuity to argue that the limit is harmonic.

Connections with complex function theory

The real and imaginary part of any holomorphic function yield harmonic functions on . Conversely, any harmonic function on an open subset of is locally the real part of a holomorphic function. This is immediately seen observing that, writing the complex function is holomorphic in because it satisfies the Cauchy–Riemann equations. Therefore, locally has a primitive, and is the real part of up to a constant, as is the real part of
Although the above correspondence with holomorphic functions only holds for functions of two real variables, harmonic functions in variables still enjoy a number of properties typical of holomorphic functions. They are analytic; they have a maximum principle and a mean-value principle; a theorem of removal of singularities as well as a Liouville theorem holds for them in analogy to the corresponding theorems in complex functions theory.

Properties of harmonic functions

Some important properties of harmonic functions can be deduced from Laplace's equation.

Regularity theorem for harmonic functions

Harmonic functions are infinitely differentiable in open sets. In fact, harmonic functions are real analytic.

Maximum principle

Harmonic functions satisfy the following maximum principle: if is a nonempty compact subset of, then restricted to attains its maximum and minimum on the boundary of. If is connected, this means that cannot have local maxima or minima, other than the exceptional case where is constant. Similar properties can be shown for subharmonic functions.

The mean value property

If is a ball with center and radius which is completely contained in the open set then the value of a harmonic function at the center of the ball is given by the average value of on the surface of the ball; this average value is also equal to the average value of in the interior of the ball. In other words,
where is the volume of the unit ball in dimensions and is the -dimensional surface measure.
Conversely, all locally integrable functions satisfying the mean-value property are both infinitely differentiable and harmonic.
In terms of convolutions, if
denotes the characteristic function of the ball with radius about the origin, normalized so that the function is harmonic on if and only if
for all x and r such that
Sketch of the proof. The proof of the mean-value property of the harmonic functions and its converse follows immediately observing that the non-homogeneous equation, for any
admits an easy explicit solution of class with compact support in. Thus, if is harmonic in
holds in the set of all points in with
Since is continuous in, converges to as showing the mean value property for in. Conversely, if is any function satisfying the mean-value property in, that is,
holds in for all then, iterating times the convolution with one has:
so that is because the -fold iterated convolution of is of class with support . Since and are arbitrary, is too. Moreover,
for all so that in by the fundamental theorem of the calculus of variations, proving the equivalence between harmonicity and mean-value property.
This statement of the mean value property can be generalized as follows: If is any spherically symmetric function supported in such that then In other words, we can take the weighted average of about a point and recover. In particular, by taking to be a function, we can recover the value of at any point even if we only know how acts as a distribution. See Weyl's lemma.

Harnack's inequality

Let
be a connected set in a bounded domain.
Then for every non-negative harmonic function,
Harnack's inequality
holds for some constant that depends only on and.

Removal of singularities

The following principle of removal of singularities holds for harmonic functions. If is a harmonic function defined on a dotted open subset of, which is less singular at than the fundamental solution, that is
then extends to a harmonic function on .

Liouville's theorem

Theorem: If is a harmonic function defined on all of which is bounded above or bounded below, then is constant.
.
Edward Nelson gave a particularly short proof of this theorem for the case of bounded functions, using the mean value property mentioned above:

Given two points, choose two balls with the given points as centers and of equal radius. If the radius is large enough, the two balls will coincide except for an arbitrarily small proportion of their volume. Since is bounded, the averages of it over the two balls are arbitrarily close, and so assumes the same value at any two points.

The proof can be adapted to the case where the harmonic function is merely bounded above or below. By adding a constant and possibly multiplying by –1, we may assume that is non-negative. Then for any two points and, and any positive number, we let We then consider the balls and where by the triangle inequality, the first ball is contained in the second.
By the averaging property and the monotonicity of the integral, we have
In the last expression, we may multiply and divide by and use the averaging property again, to obtain
But as the quantity
tends to 1. Thus, The same argument with the roles of and reversed shows that, so that
Another proof uses the fact that given a Brownian motion in such that we have for all. In words, it says that a harmonic function defines a martingale for the Brownian motion. Then a probabilistic coupling argument finishes the proof.

Generalizations

Weakly harmonic function

A function is weakly harmonic if it satisfies Laplace's equation
in a weak sense. A weakly harmonic function coincides almost everywhere with a strongly harmonic function, and is in particular smooth. A weakly harmonic distribution is precisely the distribution associated to a strongly harmonic function, and so also is smooth. This is Weyl's lemma.
There are other weak formulations of Laplace's equation that are often useful. One of which is Dirichlet's principle, representing harmonic functions in the Sobolev space as the minimizers of the Dirichlet energy integral
with respect to local variations, that is, all functions such that holds for all or equivalently, for all