Carlson's theorem


In mathematics, in the area of complex analysis, Carlson's theorem is a uniqueness theorem which was discovered by Fritz David Carlson. Informally, it states that two different analytic functions which do not grow very fast at infinity can not coincide at the integers. The theorem may be obtained from the Phragmén–Lindelöf theorem, which is itself an extension of the maximum-modulus theorem.
Carlson's theorem is typically invoked to defend the uniqueness of a Newton series expansion. Carlson's theorem has generalized analogues for other expansions.

Statement

Assume that satisfies the following three conditions. The first two conditions bound the growth of at infinity, whereas the third one states that vanishes on the non-negative integers.
  1. is an entire function of exponential type, meaning that for some real values,.
  2. There exists such that
  3. for every non-negative integer.
Then is identically zero.

Sharpness

First condition

The first condition may be relaxed: it is enough to assume that is analytic in, continuous in, and satisfiesfor some real values,.

Second condition

To see that the second condition is sharp, consider the function. It vanishes on the integers; however, it grows exponentially on the imaginary axis with a growth rate of, and indeed it is not identically zero.

Third condition

A result due to relaxes the condition that vanish on the integers. Namely, Rubel showed that the conclusion of the theorem remains valid if vanishes on a subset of upper density 1, meaning that
This condition is sharp, meaning that the theorem fails for sets of upper density smaller than 1.

Applications

Suppose is a function that possesses all finite forward differences. Consider then the Newton serieswhere is the binomial coefficient and is the -th forward difference. By construction, one then has that for all non-negative integers, so that the difference. This is one of the conditions of Carlson's theorem; if obeys the others, then is identically zero, and the finite differences for uniquely determine its Newton series. That is, if a Newton series for exists, and the difference satisfies the Carlson conditions, then is unique.