Essential singularity

Formally, consider an open subset U of the complex plane C, an element a of U and a holomorphic function f defined on U - {a}. The point a is called an essential singularity for f if it is neither a pole nor a removable singularity.

For example, the function f(z) = exp(1/z) has an essential singularity at a = 0.

The point a is an essential singularity if and only if the limit lim_z→a f(z) does not exist as a complex number nor equals infinity. This is the case if and only if the Laurent series of f at the point a has infinitely many negative degree terms.

The behavior of holomorphic functions near essential singularities is described by the Weierstrass-Casorati theorem and by the considerably stronger Picard's great theorem[?]. The latter says that in every (tiny) neighborhood of a, the function f takes on every complex value, except possibly one, infinitely often.