How to Expand a Complex Function into a Laurent Series?

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    Laurent series Series
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Discussion Overview

The discussion revolves around expanding the complex function $\frac{1}{z^2+1}$ into a Laurent series centered at $z=-i$, within the annulus defined by {$z\in ℂ|0<|z+i|<2$}. Participants explore various approaches and techniques for performing this expansion, including the use of geometric series.

Discussion Character

  • Exploratory
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • One participant expresses uncertainty about how to start the expansion and suggests that a geometric series might be applicable later.
  • Another participant provides a method to rewrite the function using partial fractions, indicating a step-by-step approach to the expansion.
  • There is a discussion about the manipulation of terms, specifically how to express $\frac{1}{z^2+1}$ in terms of its factors $(z+i)(z-i)$ and how to derive the series from that form.
  • Participants question each other's steps, particularly regarding the transformation of terms and the use of series expansions.
  • One participant attempts to rewrite the function in a different form and seeks clarification on how to consolidate the series into a single summation.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the best method for the expansion, and multiple approaches are presented. There is ongoing clarification and questioning of the steps taken by others, indicating that the discussion remains unresolved.

Contextual Notes

Some participants express confusion over specific mathematical manipulations and the need to summarize terms into a single summation, highlighting potential gaps in understanding the series expansion process.

aruwin
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Hello.

I am stuck on this question.
Let {$z\in ℂ|0<|z+i|<2$}, expand $\frac{1}{z^2+1}$ where its center $z=-i$ into Laurent series.

I have no idea how to start.
I guess geometric series could be applied later but I don't know how to start.
 
Last edited:
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aruwin said:
Hello.

I am stuck on this question.
Let {$z\in ℂ|0<|z+i|<2$}, expand $\frac{1}{z^2+1}$ where its center $z=-i$ into Laurent series.

I have no idea how to start.
I guess geometric series could be applied later but I don't know how to start.

Your start-up is very good!... You can proceed as follows...

$\displaystyle \frac{1}{1 + z^{2}} = \frac{1}{(z+i)\ (z-i)} = - \frac{\frac{1}{2}}{z+i} + \frac{\frac{1}{2}}{z-i} = $

$\displaystyle = - \frac{\frac{1}{2}}{z+i} - \frac{1}{4\ i}\ \frac{1}{1 - \frac{z+i}{2 \ i}} = - \frac{\frac{1}{2}}{z+i} - \frac{1}{4\ i}\ \sum_{n=0}^{\infty} (\frac{z+i}{2\ i})^{n}\ (1)$

Kind regards

$\chi$ $\sigma$
 
chisigma said:
$\displaystyle = - \frac{\frac{1}{2}}{z+i} - \frac{1}{4\ i}\ \frac{1}{1 - \frac{z+i}{2 \ i}} $
Kind regards
How did you change the equation into this?
 
aruwin said:
How did you change the equation into this?

... is ...

$\displaystyle \frac{\frac{1}{2}}{z-i} = \frac{\frac{1}{2}}{z + i - 2\ i}= - \frac{\frac{1}{4\ i}} {1 - \frac{z+i}{2 \ i}} $

Kind regards

$\chi$ $\sigma$
 
chisigma said:
... is ...

$\displaystyle \frac{\frac{1}{2}}{z-i} = \frac{\frac{1}{2}}{z + i - 2\ i}= - \frac{\frac{1}{4\ i}} {1 - \frac{z+i}{2 \ i}} $

Kind regards

$\chi$ $\sigma$

Oh, you divide everything by 2i, right?
 
chisigma said:
Your start-up is very good!... You can proceed as follows...

$\displaystyle \frac{1}{1 + z^{2}} = \frac{1}{(z+i)\ (z-i)} = - \frac{\frac{1}{2}}{z+i} + \frac{\frac{1}{2}}{z-i} = $

$\displaystyle = - \frac{\frac{1}{2}}{z+i} - \frac{1}{4\ i}\ \frac{1}{1 - \frac{z+i}{2 \ i}} = - \frac{\frac{1}{2}}{z+i} - \frac{1}{4\ i}\ \sum_{n=0}^{\infty} (\frac{z+i}{2\ i})^{n}\ (1)$

Kind regards

$\chi$ $\sigma$

I re-write the function this way, is this ok?
$$\frac{1}{z+i}\times\frac{1}{(z+i)-2i}$$
$$=\frac{1}{z+i}\times[\frac{\frac{1}{(-2i)}}{\frac{-(z+i)}{2i}+1}]$$
$$=\frac{1}{z+i}\times[\frac{1}{(-2i)}\times\frac{1}{1-\frac{(2+i)}{2i}}]$$

$$=\frac{1}{z+i}\times\frac{1}{(-2i)}\times\sum_{n=0}^{\infty}\left(\frac{z+i}{2i}\right)^{\!{n}}$$

I don't know how to summarize all the terms. I need to use only one ∑ in the final answer. How to do that?
 

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