Tag Info

This tag is for questions about finding a Laurent series of functions and their convergence. The Laurent series is a generalisation of the power series which allows negative indices and is essential for investigating the behaviour of functions near poles.

Laurent series: Suppose that $$~f(z)~$$ is analytic on the annulus $$~A : r_1 <|z − z_0| < r_2~$$. Then $$~f(z)~$$ can be expressed as a series $$f(z) = \sum_{n=1}^{\infty}\frac{b_n}{(z-z_0)^n}+\sum_{n=0}^{\infty}a_n(z-z_0)^n$$ The coefficients have the formulas $$a_n=\frac{1}{2\pi i}\int_\gamma \frac{f(w)}{(w-z_0)^{n+1}}\, dw$$and$$b_n=\frac{1}{2\pi i}\int_\gamma {f(w)}{(w-z_0)^{n-1}}\, dw$$where $$~\gamma~$$ is any circle $$~|w − z_0| = r~$$ inside the annulus, i.e. $$~r_1 < r < r_2~.~$$

The entire series is called the Laurent series for $$~f~$$ around $$~z_0~$$.

Notes: $$~(a)~~~$$ The series $$\sum_{n=0}^{\infty}a_n(z-z_0)^n$$ is called the analytic or regular part of the Laurent series.

$$(b)~~~$$ The series $$\sum_{n=1}^{\infty}\frac{b_n}{(z-z_0)^n}$$ is called the singular or principal part of the Laurent series.

$$(c)~~~$$ Since $$~f(z)~$$ may not be analytic (or even defined) at $$~z_0~$$ we don’t have any formulas for the coefficients using derivatives.

Remarks:

• The series $$\sum_{n=0}^{\infty}a_n(z-z_0)^n$$converges to an analytic function for $$~|z − z_0| < r_2~$$.
• The series $$\sum_{n=1}^{\infty}\frac{b_n}{(z-z_0)^n}$$converges to an analytic function for $$~ |z − z_0| > r_1~$$.
• Together, the series both converge on the annulus $$~A~$$ where $$~f~$$ is analytic.

The Laurent series is calculated over contour integrals of counterclockwise self-avoiding rectifiable paths of the function. For holomorphic functions the Taylor series and Laurent series are identical.

The Laurent series has a principal part, which consists entirely of negative-degree terms. When the principal part vanishes (there are no negative indices) the function is holomorphic; when it is an infinite sum the function has an essential pole.

Reference:

https://en.wikipedia.org/wiki/Laurent_series