Help Needed - Region of Convergence for Laurent Series of f(z)

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Discussion Overview

The discussion revolves around finding the region of convergence for the Laurent series of the function f(z) = 1/[(z^3)*cos(z)], specifically around the point z = 0. Participants explore the conditions under which the series converges and the implications of analyticity in complex analysis.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Homework-related

Main Points Raised

  • One participant expresses difficulty in determining the region of convergence for the Laurent series of the given function.
  • Another participant questions the conditions under which the function must be analytic for the series to converge, suggesting that the nature of the domain is crucial.
  • A participant clarifies that the series converges until it reaches points where the function is not analytic, identifying specific points where the denominator becomes zero (z = 0 and odd multiples of π/2).
  • It is proposed that the Laurent series around z = 0 converges for the region 0 < |z| < π/2.
  • A later reply indicates a participant's realization of the theoretical background necessary for understanding the convergence.

Areas of Agreement / Disagreement

Participants generally agree on the importance of analyticity for convergence but do not reach a consensus on the specifics of the region of convergence, as different interpretations of the function's behavior are presented.

Contextual Notes

The discussion highlights the dependence on the function's analyticity and the specific points where it fails, which are critical for determining the convergence region. However, the exact implications of these points on the convergence are not fully resolved.

Who May Find This Useful

This discussion may be useful for students and practitioners in complex analysis, particularly those interested in Laurent series and the conditions for convergence in complex functions.

Butelle
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Hello -

I have a problem in general finding the region in which the Laurent series converges...

Could someone please help me with this question - I know that this is is meant to be easy (as there is no fully worked solution to this) but I don't understand it:

f(z) = 1/ [(z^3)*cosz]. The function has a Laurent series about z = 0 converging at Pi/4. What is the region in which this Laurent series converges?

Thanks.
 
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When your textbook derived the Laurent expansion, what was the shape of the domain that the function had to be analytic in (it is complex analysis after all - isn't everything about being analytic?). What can cause a function to not have that property, and hence will make the series not converge?
 
I'm sorry I am not really understanding? I derived the expansion and then I didn't know what to do after that?
 
He is saying that the series will converge until it hits a point at which the function is NOT analytic. This particular function, f(z)= 1/(z3cos(z)), is analytic except where the denominator is 0: z= 0 or z an odd multiple of [itex]\pi/2[/itex]. That is, the Laurent series around z= 0 converges for 0< |z|< [itex]\pi/2[/itex].
 
ahhh ok i understand - see i didnt even understand the theory behind it! i get it now - thanks! :)
 

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