# Casorati–Weierstrass theorem

In complex analysis, a branch of mathematics, the **Casorati–Weierstrass theorem** describes the behaviour of holomorphic functions near their essential singularities. It is named for Karl Theodor Wilhelm Weierstrass and Felice Casorati. In Russian literature it is called Sokhotski's theorem.

## Formal statement of the theorem

Start with some open subset in the complex plane containing the number , and a function that is holomorphic on , but has an essential singularity at . The *Casorati–Weierstrass theorem* then states that

- if is any neighbourhood of contained in , then is dense in .

This can also be stated as follows:

- for any , and complex number , there exists a complex number in with and .

Or in still more descriptive terms:

- comes arbitrarily close to
*any*complex value in every neighbourhood of .

The theorem is considerably strengthened by Picard's great theorem, which states, in the notation above, that assumes *every* complex value, with one possible exception, infinitely often on .

In the case that is an entire function and , the theorem says that the values approach every complex number and , as tends to infinity. It is remarkable that this does not hold for holomorphic maps in higher dimensions, as the famous example of Pierre Fatou shows.[1]

## Examples

The function *f*(*z*) = exp(1/*z*) has an essential singularity at 0, but the function *g*(*z*) = 1/*z*^{3} does not (it has a pole at 0).

Consider the function

This function has the following Taylor series about the essential singular point at 0:

Because exists for all points *z* ≠ 0 we know that *ƒ*(*z*) is analytic in a punctured neighborhood of *z* = 0. Hence it is an isolated singularity, as well as being an essential singularity.

Using a change of variable to polar coordinates our function, *ƒ*(*z*) = *e*^{1/z} becomes:

Taking the absolute value of both sides:

Thus, for values of *θ* such that cos *θ* > 0, we have as , and for , as .

Consider what happens, for example when *z* takes values on a circle of diameter 1/*R* tangent to the imaginary axis. This circle is given by *r* = (1/*R*) cos *θ*. Then,

and

Thus, may take any positive value other than zero by the appropriate choice of *R*. As on the circle, with *R* fixed. So this part of the equation:

takes on all values on the unit circle infinitely often. Hence *f*(*z*) takes on the value of every number in the complex plane except for zero infinitely often.

## Proof of the theorem

A short proof of the theorem is as follows:

Take as given that function *f* is meromorphic on some punctured neighborhood *V* \ {*z*_{0}}, and that *z*_{0} is an essential singularity. Assume by way of contradiction that some value *b* exists that the function can never get close to; that is: assume that there is some complex value *b* and some ε > 0 such that |*f*(*z*) − *b*| ≥ ε for all *z* in *V* at which *f* is defined.

Then the new function:

must be holomorphic on *V* \ {*z*_{0}}, with zeroes at the poles of *f*, and bounded by 1/ε. It can therefore be analytically continued (or continuously extended, or holomorphically extended) to *all* of *V* by Riemann's analytic continuation theorem. So the original function can be expressed in terms of *g*:

for all arguments *z* in *V* \ {*z*_{0}}. Consider the two possible cases for

If the limit is 0, then *f* has a pole at *z*_{0} . If the limit is not 0, then *z*_{0} is a removable singularity of *f* . Both possibilities contradict the assumption that the point *z*_{0} is an essential singularity of the function *f* . Hence the assumption is false and the theorem holds.

## History

The history of this important theorem is described by
Collingwood and Lohwater.[2]
It was published by Weierstrass in 1876 (in German) and by Sokhotski in 1868 in his Master thesis (in Russian).
So it was called Sokhotski's theorem in the Russian literature and Weierstrass's theorem in
the Western literature.
The same theorem was published by Casorati in 1868, and
by Briot and Bouquet in the *first edition* of their book (1859).[3]
However, Briot and Bouquet *removed* this theorem from the second edition (1875).

## References

- Fatou, P. (1922). "Sur les fonctions meromorphes de deux variables".
*Comptes rendus*.**175**. pp. 862, 1030. - Collingwood, E; Lohwater, A (1966).
*The theory of cluster sets*. Cambridge University Press. - Briot, Ch; Bouquet, C (1859).
*Theorie des fonctions doublement periodiques, et en particulier, des fonctions elliptiques*. Paris.

- Section 31, Theorem 2 (pp. 124–125) of Knopp, Konrad (1996),
*Theory of Functions*, Dover Publications, ISBN 978-0-486-69219-7