Harmonic Function

Harmonic functions that arise in the subject physics are determined by their singularities as well as boundary conditions (such as Dirichlet boundary conditions or Neumann boundary conditions). On regions without boundaries, adding the real part or the imaginary part of an entire function will produce a harmonic function with the same singularity, so in this case, the harmonic function can not be 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.

Harmonic functions appear regularly and these functions play a fundamental role in math, physics, as well as in engineering. In this article, we are going to learn the definition, some key properties.

What is Harmonic Function? 

Let’s start by defining harmonic functions and looking at some of the properties of harmonic functions. 

A function u(x, y) is known as harmonic if it is twice continuously differentiable and satisfies the following partial differential equation:

\[ \nabla ^{2} u = u_{xx} + u_{yy} = 0 \]. (1) Equation 1 is called Laplace’s equation. 

So a function is known to be harmonic if it satisfies Laplace’s equation. 

Operator \[\nabla ^{2}\]  is known as the Laplacian and \[\nabla ^{2} u \] is known as the Laplacian of u. 

(Image will be uploaded soon)

What is Conjugate Harmonic Function?

If f(z) = u+iv is known to be an analytic function of z, then v is known as a conjugate harmonic function of u, and u in its turn is termed as a conjugate harmonic function of v.

Or u and v are known as conjugate harmonic functions.

What are Spherical Harmonic Functions?

Spherical harmonic functions generally arise when the spherical coordinate system is used. (In this system, a point in space is located by 3 coordinates, out of which one representing the distance from the origin and two others representing the angles of elevation and azimuth, as in astronomy.) The spherical harmonic functions are commonly used to describe three-dimensional fields, such as they are used to describe magnetic, gravitational, as well as electrical fields.

Why are Harmonic Functions Called Harmonic?

"Harmonic" known to be the descriptor in the name harmonic function, generally originates from a point on a taut string that is undergoing harmonic motion. The solution to the differential equation for this type of motion can be written in terms of sines as well as in terms of cosines, functions which are thus referred to as harmonics.

What are Positive Harmonic Functions?

The Poisson integral formula allows obtaining useful inequalities for positive harmonic functions. Note: It is important to keep in mind that if a non-negative harmonic function attains a minimum value of 0 on a domain, it is 0 throughout the domain. So the class of non-negative harmonic functions on a domain basically consists of all positive functions as well as a zero function.

Condition for Harmonic Function

What Makes a Function Harmonic?

A real-valued function of a single variable is harmonic precisely when it has the form f(x)=ax+b  for some numbers a,b, so its graph is a straight line. You can “see” that a graph is a straight line if you have a vision of infinite precision and infinite breadth (you need to make sure the graph doesn’t take a nosedive at  x= \[10^{7000}\]), but assuming that straight lines are visibly straight lines then you can do that.

The graph of a function f: \[ R^{2}\rightarrow {R} \]  is still something we can visualize – as a surface, or using contour lines – but the harmonic nature of the graph is not something you can “see”. You need the sum of the two-second derivatives to be equal to 0 everywhere, and that doesn’t dictate any easily recognizable shape.

Linear functions or affine functions are still harmonic, and you can recognize their surface graphs as planes, but many functions that aren’t affine are also harmonic. For example,  f(x,y)= e^xcos(y)  is harmonic, but e^xcos(1.1y)  is not. 

Can you Visually Tell Them Apart?

You can sometimes recognize that a function is not harmonic by noticing that it has local maxima or local minima. Harmonic functions can’t have such extremal values.

The value of a harmonic function at any point is the average of its values on a sphere (in two variables, for example, a circle) centered at that point.

Harmonic functions of more than 2 variables pose an even more serious visualization challenge since their graphs no longer fit in three dimensions.

Problems to be Solved

Question 1) Verify u(x,y) =\[x^{3} - 3xy^{2} - 5y\] is harmonic in the entire complex plane.

Solution) \[\frac{\delta u}{\delta x}\] =\[3x^{2} - 3y^{2}\], \[\frac{\delta^{2} u}{\delta x^{2}}\]= 6x, \[\frac{\delta u}{\delta y}\]= -6xy-5, \[\frac{\delta^{2} u}{\delta y^{2}}\] = -6x

Therefore,\[ \frac{\delta^{2} u}{\delta x^{2}} + \frac{\delta^{2} u}{\delta y^{2}} \]= 6x-6x = 0