ASL STEM

Definition

Let U subset= C be a domain, and let f be an analytic function on U. Then if there is a point z_0 in U such that |f(z_0)|>=|f(z)| for all z in U, then f is constant. The following slightly sharper version can also be formulated. Let U subset= C be a domain, and let f be an analytic function on U. Then if there is a point z_0 in U at which |f| has a local maximum, then f is constant. Furthermore, let U subset= C be a bounded domain, and let f be a continuous function on the closed set U^_ that is analytic on U. Then the maximum value of |f| on U^_ (which always exists) occurs on the boundary partialU. In other words, max_(U^_)|f|=max_(partialU)|f|. The maximum modulus theorem is not always true on an unbounded domain.

Source: http://mathworld.wolfram.com