All Laurent series expansion around 1.

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Homework Help Overview

The problem involves finding all Laurent series expansions of the function f(z) = z^4/(3 + z^2) around the point z = 1. Participants are exploring the implications of singularities and the regions in which the Laurent series should be computed.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • The original poster attempts to substitute z with (u + 1) to reframe the function in terms of u, leading to the expression f(u + 1) = (u + 1)^4/(u^2 + 2u + 4). They express confusion about determining the regions for the Laurent series and the number of series that may exist.
  • Some participants suggest considering the poles of the transformed function to define the annuli for the Laurent series.
  • Others propose manipulating the expression to separate it into a polynomial and a term suitable for geometric series expansion.

Discussion Status

The discussion is active, with participants providing guidance on how to visualize the problem by plotting poles and defining annuli. There is an exploration of different approaches to manipulate the function for series expansion, but no consensus has been reached on the exact conditions or methods to apply.

Contextual Notes

Participants are considering the implications of singularities and the specific regions around z = 1 where the Laurent series may be valid. There is an acknowledgment of the complexity involved in determining these regions based on the function's behavior.

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Homework Statement



Question is= Find all Laurent series expansion of f(z)=z^4/(3+z^2) around 1. I will be very very thankful if someone can help me to do this question.

Homework Equations





The Attempt at a Solution



can I assume (z-1=u) here and change the function in terms of $u$. then i will have

f(u+1)=(u+1)^4/(u+2)^2

It should have singularity at $u=-2$. now I m big confuse about in what regions I should compute Laurent series.

I have trouble of thinking that how many laurent series I will have and what will be the conditions on $z$.

Thanks !
 
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first, subbing in for z = u+1
g(u) = f(u+1)=(u+1)^4/(3+(u+1)^2) =(u+1)^4/(u^2+ 2u +4)
 
there will be a laurent series for each disk about z-1 = u = 0 defined by the poles of the function
 
You've done the substitution to center the Laurent expansion around u=0, now draw a diagram, plotting the poles of the new function and draw the annuli these poles split the plane into. For each annuli, decide what the rule is for that region (eg 8< |u| < 23 or something).

The next part is a bit harder: Try to manipulate the expression you have ( [tex]\frac{ (u+1)^4}{(u+1)^2+3}[/tex] ) into the product of two terms, 1 a polynomial, and another that you can interpret as the sum of a geometric series. Then you can expand the geometric series into a sum, and multiply the polynomial into it to get the Laurent series.
 

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