HARMONIC FUNCTION

mathematical function of two variables having the property that its value at any point is equal to the average of its values along any circle around that point, provided the function is defined within the circle. An infinite number of points are involved in this average, so that it must be found by means of an integral, which represents an infinite sum. In physical situations, harmonic functions describe those conditions of equilibrium such as the temperature or electrical charge distribution over a region in which the value at each point remains constant. Harmonic functions can also be defined as functions that satisfy Laplace's equation, a condition that can be shown to be equivalent to the first definition. The surface defined by a harmonic function has zero convexity, and these functions thus have the important property that they have no maximum or minimum values inside the region in which they are defined. Harmonic functions are also analytic, which means that they possess all derivatives (are perfectly smooth) and can be represented as polynomials with an infinite number of terms, called power series. Spherical harmonic functions arise when the spherical coordinate system (a system that locates a point in space by coordinates in which one, r, represents distance from the origin and two others, q and f, represent the angles of elevation and azimuth, as in astronomy) is used in investigating physical problems in three dimensions involving fields such as gravitational, magnetic, electrical, and those arising from certain types of fluid motion. There are two types of spherical harmonics: (1) solid spherical harmonics, Rn(x, y, z), which are special nth-degree polynomials having a value for all points inside a sphere; and (2) surface spherical harmonics, Sn(q, f), which describe a function only on the surface of a sphere. These two types of harmonics are related by the equation Rn(x, y, z) = rnSn(q, f), and it is the determination of the surface spherical harmonic that thus represents the difficult part of the theory. The surface spherical harmonic Sn(q, f) can be written as the product of two functions, one of which is exp(imf), in which m is a positive integer or zero, and another, more complicated function that depends only on q. The part that depends on q is called an associated Legendre function, and these functions are the solutions of a differential equation called the associated Legendre equation. Any solution of the Laplace equation can be written as a (possibly infinite) sum of solid spherical harmonics.