# Harmonic functions

1. Nov 26, 2014

### Fosheimdet

If $$f(z)=u(x,y)+iv(x,y)$$ is analytic in a domain D, then both u and v satisfy Laplace's equations
$$\nabla^2 u=u_{xx} + u_{yy}=0$$
$$\nabla^2 v=v_{xx} + v_{yy}=0$$

and u and v are called harmonic functions.

My question is whether or not this goes both ways. If you have two functions u and v which satisfy the Laplace equations are they the real and imaginary parts of an analytic function?

2. Nov 26, 2014

### ShayanJ

Just take the real part of one function and the imaginary part of another function. They satisfy Laplace's equation but aren't the real and imaginary parts of a function because they don't satisfy the Cauchy-Riemann conditions.

3. Nov 28, 2014

### mathwonk

A Shyan said, the answer to your question is no, since the real part of a holomorphic function determines the imaginary part. But every harmonic function is locally the real part of a holomorphic function, although not necessarily globally, due to branching behavior that may occur only in the imaginary part. E.g. log(|z|), defined everywhere but z=0, is the real part of log(z), but the imaginary part of log(z), a multiple of arg(z), is only defined locally near non zero values of z.

4. Nov 28, 2014

### WWGD

In yet another way, the Real part of a holomorphic function can only have ( up to a difference by a constant) one
Complex counterpart, so the odds are that two harmonic functions are respectively the Real and Imaginary part of a holomorphic function.