Laurent series of the function f(z)=1/z^2 for |z-a|>|a|

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Discussion Overview

The discussion revolves around finding the Laurent series for the function f(z) = 1/z^2 in the region where |z-a| > |a|, with a specified that a is not equal to zero. Participants explore various methods and approaches to derive the series, including the use of geometric series and integration techniques.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • One participant expresses frustration with the problem and seeks help for the Laurent series of f(z) = 1/z^2.
  • Another participant suggests using a formula for the coefficients that involves integration over the annulus defined by |z - a| > |a|.
  • A different participant proposes rewriting 1/z^2 to resemble a geometric series in 1/(z-a) but later questions the feasibility of this approach.
  • One participant mentions that differentiating -1/z yields 1/z^2, implying that a Laurent series for -1/z could be useful.
  • Another participant acknowledges a potential solution and expresses relief but then shifts focus to a different function, F(z) = exp(z + 1/z), questioning how to handle the multiplication of series for e^z and e^(1/z).
  • One participant comments on the perceived difficulty of multiplying series and notes that each coefficient will be an infinite sum, suggesting there might be a clever method to simplify this.
  • A later reply questions the starting point for the limits in the series multiplication, indicating uncertainty about the mechanics involved.
  • Another participant suggests that relevant theorems from an analysis or advanced calculus course should assist in understanding the series, emphasizing that the series are absolutely convergent.

Areas of Agreement / Disagreement

Participants express varying degrees of confidence and uncertainty regarding the methods for finding the Laurent series. There is no consensus on the best approach, and multiple competing views on how to tackle the problem remain present.

Contextual Notes

Participants mention the need for specific techniques and formulas, indicating that the discussion is dependent on prior knowledge of series and convergence, which may not be universally understood among all participants.

heman
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Hi all,,

I have an Exam tommorow and this question is irritating me...Pls help

Laurent series of the function f(z)=1/z^2 for |z-a|>|a| .a is not equal to zero...
I am waiting for yours responses...I will be highly thankful to you.
 
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Don't you have a formula for the coefficients, that involves integrating over the annulus |z - a| > |a|?

Or, maybe you could look at it as:

|1 / (z-a)| < 1/|a|

And try to rewrite 1/z^2 so it looks like the sum of a geometric series in 1/(z-a)?
 
Omg,,,I did not use the formula for the coefficients,it did not seemed to strike me,,,yeah then its easy...and can be solved...

I was trying to do it by bringing in the form of geometric series..
Hurkyl,
Pls tell me how can i write 1/z^2 in that apt form as here the power of z is 2...
I seem to moving forward but then 2 series will be multiplied acc. to me,won't it be tedious..?
 
Hrm, yah, writing it as a geometric series won't work... but maybe the sum of two geometric series?

I guess it would be easier to observe that d/dz (-1/z) = 1/z^2, so if you had a Laurent series for -1/z...
 
Oh yes,,,i got it ...That will work...Thanks Hurkyl ,I feel much better now!

But in writing the Laurentz Series for F(z)=exp(z+1/z) around zero ,i think i have got no escape ...i think i am bound to multiply the series for e^z and e^(1/z),,,But how will i write the coefficient for each term,,or there can be any better approach...
 
I don't see the difficulty in multiplying the series... I guess you don't like the fact that each coefficient will be an infinite sum. :smile: (Though, it wouldn't surprise me if there's a clever way to do this that gives you a closed form)
 
Hurkyl said:
I don't see the difficulty in multiplying the series... I guess you don't like the fact that each coefficient will be an infinite sum. :smile: (Though, it wouldn't surprise me if there's a clever way to do this that gives you a closed form)


"Each coefficient will be an infinite sum."...that i can see but where it will start that seems out of my reach...limits are worrying me...i am able to see it mechanically only...
I think i sincerely need help on this!
 
Have you taken an analysis or advanced calculus course? The relevant theorems should be in there.

The main thing here, though, is that the series involved are absolutely convergent, meaning that you can rearrange terms in any way you please.
 

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