Maximum modulus principle

Statement

Suppose $U\subset \mathbb {C}$ is a domain (open connected subset). Let $f:U\to \mathbb {C}$ be a holomorphic function. The maximum modulus principle (sometimes called the maximum principle) states that if there exists a $z_{0}\in U$, such that for all $z\in U$, we have:

$|f(z)|\leq |f(z_{0})|$

Then, $f$ is a constant function.

Facts used

• Open mapping theorem

Proof

Suppose $f$ is a nonconstant holomorphic function on a nonempty domain $U$. We'll show that $f$ cannot have a maximum.

First, by the open mapping theorem, $f$ is an open map.

Also, observe that the map $|\cdot |:\mathbb {C} \to [0,\infty )$ is an open map. Thus, the composite map $|f|:U\to [0,\infty )$ given by $z\mapsto |f(z)|$ is also an open map. Thus, under this map, the image of $U$ must be an open connected subset of $[0,\infty )$ so it must be of the form $[0,a)$ where $a\in \mathbb {R}$ or $a=\infty$. Hence, there cannot be a maximum within $U$.