In complex analysis, a pole of a function is a certain type of simple singularity that behaves like the singularity of f(z) = 1/zn at z = 0; a pole of a function f is a point a such that f(z) approaches infinity as z approaches a.
Formally, suppose U is an open subset of the complex plane C, a is an element of U and f : U − {a} → C is a holomorphic function. If there exists a holomorphic function g : U → C and a natural number n such that
for all z in U − {a}, then a is called a pole of f. If n is chosen as small as possible, then n is called the order of the pole.
The number a is a pole of order n of f if and only if the Laurent series expansion of f around a has only finitely many negative degree terms, starting with (z - a)−n.
A pole of order 0 is a removable singularity. In this case the limit limz→a f(z) exists as a complex number. If the order is bigger than 0, then limz→a f(z) = ∞.
If the first derivative of a function f has a pole of order 1 (a "simple" pole) at a, then a is a branch point of f.
A non-removable singularity that is not a pole or a branch point is called an essential singularity.
A holomorphic function whose only singularities are poles is called meromorphic.
See also