Isolated singular point
for an element of an analytic function
A point a in the complex z - plane satisfying the following properties: 1) the element of f ( z) does not have an analytic continuation along any path to a ; and 2) there exists a number R > 0 such that analytic continuation of f ( z) is possible along any path in the punctured neighbourhood U = \{ {z \in \mathbf C } : {0 < | z - a | < R } \} of a .
If a new element is obtained when f ( z) is continued analytically along a closed path in U encircling a , for example along the circle | z - a | = \rho , 0 < \rho < R , then a is called a branch point, or an isolated singular point of multi-valued character. Otherwise the element of f ( z) defines a single-valued analytic function in U and a is called an isolated singular point of single-valued character. In a punctured neighbourhood U of an isolated singular point a of single-valued character, f ( z) can be expanded in a Laurent series:
\tag{1 } f ( z) = \ \sum _ {k = - \infty } ^ { {+ } \infty } c _ {k} ( z - a) ^ {k}
with regular part f _ {1} ( z) = \sum _ {k = 0 } ^ {+ \infty } c _ {k} ( z - a) ^ {k} and principal part f _ {2} ( z) = \sum _ {k = - \infty } ^ {-} 1 c _ {k} ( z- a) ^ {k} . The behaviour of an analytic function f ( z) in a punctured neighbourhood U of an isolated singular point of single-valued character is determined, in principle, by the principal part of its Laurent series. If all the coefficients of the principal part are zero, then on setting f ( a) = c _ {0} one gets a single-valued analytic function in a full neighbourhood of a . This case of practical absence of a singularity is also characterized by the fact that f ( z) is bounded in U , or by the fact that the limit \lim\limits _ {z \rightarrow a } f ( z) = c _ {0} , z \in U , exists and is finite.
If among the coefficients of the principal part only finitely many are non-zero, and that with smallest index is c _ {-} m \neq 0 , then a is a pole of order m ( cf. Pole (of a function)). A pole a is also characterized by the fact that
\lim\limits _ {z \rightarrow a } f ( z) = \infty ,\ \ z \in U.
Finally, if there are infinitely many non-zero coefficients in the principal part, then a is an essential singular point. In this case the following limit does not exist, neither finite nor infinite:
\lim\limits _ {z \rightarrow a } f ( z),\ z \in U.
For an isolated singular point a = \infty at infinity of the element f( z) , a punctured neighbourhood has the form U = \{ {z \in \mathbf C } : {r < | z | < \infty } \} , and the Laurent series is
f ( z) = \ \sum _ {k = - \infty } ^ { {+ } \infty } c _ {k} z ^ {k} .
Here the regular part is f _ {1} = \sum _ {k = - \infty } ^ {0} c _ {k} z ^ {k} and the principal part is f _ {2} ( z) = \sum _ {k = 1 } ^ {+ \infty } c _ {k} z ^ {k} . With these conditions, the above descriptions of the classification and criteria for the type of an isolated singular point carry over to the case a = \infty without further change (see also Residue of an analytic function). It should be noted that the elements of different branches of the complete analytic function f ( z) at one and the same point a \in \mathbf C may have singularities of completely-different types.
Holomorphic functions f ( z) of several complex variables z = ( z _ {1} \dots z _ {n} ) , n \geq 2 , cannot have isolated singular points. For n \geq 2 , the singular points form infinite sets of singularities.
References
[1] | A.I. Markushevich, "Theory of functions of a complex variable" , 1 , Chelsea (1977) (Translated from Russian) MR0444912 Zbl 0357.30002 |
[2] | B.V. Shabat, "Introduction of complex analysis" , 1–2 , Moscow (1976) (In Russian) Zbl 0799.32001 Zbl 0732.32001 Zbl 0732.30001 Zbl 0578.32001 Zbl 0574.30001 |
Comments
References
[a1] | E.C. Titchmarsh, "The theory of functions" , Oxford Univ. Press (1979) MR0593142 MR0197687 MR1523319 Zbl 0477.30001 Zbl 0336.30001 Zbl 0005.21004 Zbl 65.0302.01 Zbl 58.0297.01 |
[a2] | L.V. Ahlfors, "Complex analysis" , McGraw-Hill (1979) pp. 241 MR0510197 MR1535085 MR0188405 MR1570643 MR1528598 MR0054016 Zbl 0395.30001 |
Isolated singular point. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Isolated_singular_point&oldid=47438