I have the question:
If $u$ is a non-constant harmonic function, it must have a zero. Where is $u$ defined from the complex to the real set?
My approach has been as follows:
Since $u$ is non-constant harmonic, it must have a harmonic conjugate $v$ such that $f = u + iv$ is analytic. For a $z$ in a circle, we can use the Cauchy Integral formula to define $f(z)$ and then separate it into real and imaginary parts.
Doing this, we get the formula from the mean value property for harmonic functions. But I am very confused about the mean value property and how it ensures a zero. Since $u$ is non-constant, does it take both negative and positive values and hence by IVP, it must take zero as well? Any pointers on how to proceed?
Best Answer
Suppose $u:\mathbb C\to \mathbb R$ is a nonconstant harmonic function, and that $u$ is never $0.$ Because $u$ is continuous, $u(\mathbb C)$ is a connected subset of $\mathbb R.$ That implies $u(\mathbb C)$ is an interval that doesn't contain $0.$ Thus either $u(\mathbb C)\subset (0,\infty)$ or $u(\mathbb C)\subset (-\infty,0).$ Suppose it's the second case. Let $v$ be a harmonic conjugate of $u$ on $\mathbb C.$ Then $f(z) = e^{u+iv}$ is an entire function such that $|f(z)| = e^{u(z)} \le e^0=1$ everywhere. This implies $f$ is a bounded entire function. By Liouville, $f$ is constant. That implies $u$ is constant, contradiction.