Essential singularity

Plot of the function exp(1/z), centered on the essential singularity at z=0. The hue represents the complex argument, the luminance represents the absolute value. This plot shows how approaching the essential singularity from different directions yields different behaviors (as opposed to a pole, which, approached from any direction, would be uniformly white).
Model illustrating essential singularity of a complex function 6w=exp(1/(6z))

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 descriptionEdit

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 descriptionsEdit

Let a be a complex number, assume that f(z) 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

    and       exist, then a is a removable singularity of both f and 1/f.


    exists but       does not exist, then a is a zero of f and a pole of 1/f.

Similarly, if

    does not exist but       exists, then a is a pole of f and a zero of 1/f.

If neither

    nor       exists, then a is an essential singularity of both f and 1/f.

Another way to characterize an essential singularity is that the Laurent series of f at the point a has infinitely many negative degree terms (i.e., the principal part of the Laurent series is an infinite sum). 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  .[1]

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. (The exception is necessary, as the function exp(1/z) never takes on the value 0.)


  1. ^ Weisstein, Eric W. "Essential Singularity". MathWorld, Wolfram. Retrieved 11 February 2014.
  • Lars V. Ahlfors; Complex Analysis, McGraw-Hill, 1979
  • Rajendra Kumar Jain, S. R. K. Iyengar; Advanced Engineering Mathematics. Page 920. Alpha Science International, Limited, 2004. ISBN 1-84265-185-4

External linksEdit