Maximum modulus principle

A plot of the modulus of cos(z) (in red) for z in the unit disk centered at the origin (shown in blue). As predicted by the theorem, the maximum of the modulus cannot be inside of the disk (so the highest value on the red surface is somewhere along its edge).

In mathematics, the maximum modulus principle in complex analysis states that if f is a holomorphic function, then the modulus | f | cannot exhibit a true local maximum that is properly within the domain of f.

In other words, either f is a constant function, or, for any point z₀ inside the domain of f there exist other points arbitrarily close to z₀ at which |f | takes larger values.

Formal statement

Let f be a function holomorphic on some connected open subset D of the complex plane C and taking complex values. If z₀ is a point in D such that

for all z in a neighborhood of z₀, then the function f is constant on D.

Sketch of the proof

One uses the equality

log f(z) = log |f(z)| + i arg f(z)

for complex natural logarithms to deduce that log |f(z)| is a harmonic function. Since z₀ is a local maximum for this function also, it follows from the maximum principle that |f(z)| is constant. Then, using the Cauchy-Riemann equations we show that f'(z)=0, and thus that f(z) is constant as well.

Another proof works by using Gauss's mean value theorem to "force" all points within overlapping open disks to assume the same value. The disks are laid such that their centers form a polygonal path from the value where f(z) is maximized to any other point in the domain, while being totally contained within the domain. Thus the existence of a maximum value implies that all the values in the domain are the same, thus f'(z) is constant.

By switching to the reciprocal, we can get the minimum modulus principle. It states that if f is holomorphic within a bounded domain D, continuous up to the boundary of D, and non-zero at all points, then the modulus |f (z)| takes its minimum value on the boundary of D.

Alternatively, the maximum modulus principle can be viewed as a special case of the open mapping theorem, which states that a holomorphic function maps open sets to open sets. If |f| attains a local maximum at a, then clearly the direct image of sufficiently small open neighborhoods of a cannot be open. Therefore, f is constant.

Applications

The maximum modulus principle has many uses in complex analysis, and may be used to prove the following:

• The fundamental theorem of algebra, as may be seen in the classic text "Introduction to Complex Analysis", by Nevanlinna and Paatero.
• Schwarz's lemma, a result which in turn has many generalisations and applications in complex analysis.
• The Phragmén-Lindelöf principle, an extension to unbounded domains.

References

• E.C. Titchmarsh, The Theory of Functions (2nd Ed) (1939) Oxford University Press. (See chapter 5.)
• E.D. Solomentsev (2001), "Maximum-modulus principle", in Hazewinkel, Michiel, Encyclopaedia of Mathematics, Kluwer Academic Publishers, ISBN 978-1556080104 

External links

• Eric W. Weisstein, Maximum Modulus Principle at MathWorld.
• The Maximum Modulus Principle by John H. Mathews