Essential singularity


In complex analysis, an essential singularity of a function is a "severe" singularity near which the function exhibits odd behavior.
The category essential singularity is a "left-over" or default group of isolated singularities that are especially unmanageable: by definition they fit into neither of the other two categories of singularity that may be dealt with in some manner - removable singularities and poles.

Formal description

Consider an open subset of the complex plane. Let be an element of, and a holomorphic function. The point is called an essential singularity of the function if the singularity is neither a pole nor a removable singularity.
For example, the function has an essential singularity at.

Alternate descriptions

Let a be a complex number, assume that f is not defined at a but is analytic in some region U of the complex plane, and that every open neighbourhood of a has non-empty intersection with U.
If both
If
Similarly, if
If neither
Another way to characterize an essential singularity is that the Laurent series of f at the point a has infinitely many negative degree terms. A related definition is that if there is a point for which no derivative of converges to a limit as tends to, then is an essential singularity of.
The behavior of holomorphic functions near their essential singularities is described by the Casorati–Weierstrass theorem and by the considerably stronger Picard's great theorem. The latter says that in every neighborhood of an essential singularity a, the function f takes on every complex value, except possibly one, infinitely many times.