Identity theorem

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In complex analysis, a branch of mathematics, the identity theorem for holomorphic functions states: given functions f and g holomorphic on a domain D (open and connected subset), if f = g on some , having an accumulation point, then f = g on D.

Thus a holomorphic function is completely determined by its values on a (possibly quite small) neighborhood in D. This is not true for real-differentiable functions. In comparison, holomorphy, or complex-differentiability, is a much more rigid notion. Informally, one sometimes summarizes the theorem by saying holomorphic functions are "hard" (as opposed to, say, continuous functions which are "soft").

The underpinning fact from which the theorem is established is the developability of a holomorphic function into its Taylor series.

The connectedness assumption on the domain D is necessary. For example, if D consists of two disjoint open set, can be on one open set, and on another, while is on one, and on another.


If two holomorphic functions f and g on a domain D agree on a set S which has an accumulation point c in D, then f = g on a disk in centered at .

To prove this, it is enough to show that for all .

If this is not the case, let m be the smallest nonnegative integer with . By holomorphy, we have the following Taylor series representation in some open neighborhood U of c:

By continuity, h is non-zero in some small open disk B around c. But then f − g ≠ 0 on the punctured set B − {c}. This contradicts the assumption that c is an accumulation point of {f = g}.

This lemma shows that for a complex number a, the fiber f−1(a) is a discrete (and therefore countable) set, unless f = a.


Define the set on which and has the same Taylor expansion:

We'll show is nonempty, open, and closed. Then by connectedness of , must be all of , which implies on .

By the lemma, in a disk centered on a disk in centered at , they have the same Taylor series at , so , is nonempty.

, because the Taylor series of and at have non-zero radius of convergence, the open disk also lies in S for some r. So S is open.

By holomorphy of and , they have holomorphic derivatives, so all are continuous, so is an intersection of closed sets, so it's closed.


  • Ablowitz, Mark J.; Fokas A. S. (1997). Complex variables: Introduction and applications. Cambridge, UK: Cambridge University Press. p. 122. ISBN 0-521-48058-2.