# Pole (of a function)

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An isolated singular point of single-valued character of an analytic function of the complex variable for which increases without bound when approaches : . In a sufficiently small punctured neighbourhood of the point , or in the case of the point at infinity , the function can be written as a Laurent series of special form:

 (1)

or, respectively,

 (2)

with finitely many negative exponents if , or, respectively, finitely many positive exponents if . The natural number in these expressions is called the order, or multiplicity, of the pole ; when the pole is called simple. The expressions (1) and (2) show that the function if , or if , can be analytically continued (cf. Analytic continuation) to a full neighbourhood of the pole , and, moreover, . Alternatively, a pole of order can also be characterized by the fact that the function has a zero of multiplicity at .

A point of the complex space , , is called a pole of the analytic function of several complex variables if the following conditions are satisfied: 1) is holomorphic everywhere in some neighbourhood of except at a set , ; 2) cannot be analytically continued to any point of ; and 3) there exists a function , holomorphic in , such that the function , which is holomorphic in , can be holomorphically continued to the full neighbourhood , and, moreover, . Here also

however, for , poles, as with singular points in general, cannot be isolated.

#### References

 [1] B.V. Shabat, "Introduction of complex analysis" , 2 , Moscow (1976) (In Russian)