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Finding a Laurent series / residue problem

  1. Mar 25, 2013 #1
    1. The problem statement, all variables and given/known data
    [itex] f(z) = \frac{1}{ \exp{ \frac {z^2 - \pi/2}{ \sqrt{3} } } + i } [/itex]

    Find the residue of f(z) at [itex] z_0 = \frac{ \sqrt(\pi) }{2 } ( \sqrt(3) - i ) [/itex]

    2. Relevant equations

    3. The attempt at a solution

    I was able to verify that the given z_0 is a singularity, and furthermore an essential singularity. However, I am stumped at how to figure out the residue (value of [itex] a_{-1} [/itex] ) from the given information.

    I've tried rearranging letting f(z) be a general Laurent series, and then rearranged the equation. I've tried making multiplicative arguments, like below:

    [itex] \sum_{n \in Z} d_n (z - z_0)^n \cdot ( \exp{ \frac {z^2 - \pi/2}{ \sqrt{3} } } + i ) = 1[/itex]

    In the past I've been able to figure out the residue from multiplicative arguments and matching powers, but I don't see how I can do that here. Since the given z_0 is an essential singularity, there will necessarily be an infinite number of negative-degree terms, which must cancel with the infinite number of positive-degree terms from the exponential portion, and it gets so messy that I can't resolve anything.

    Furthermore, I am troubled by the fact that I would need an infinite number of negative powers to cancel with an infinite number of positive powers, and so I am not even sure if the usual approach of matching powers will terminate or give an answer.

    Can anyone provide a hint, or point out an error I may have made?

    I should mention that I've found series for the exponential expression and trivially for the 1, both about z0. But I'm not finding a nice pattern for the coefficients of the exponential so that I can equate coefficients. The first few terms I have but they're not pretty
    Last edited: Mar 25, 2013
  2. jcsd
  3. Mar 25, 2013 #2


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    It doesn't look like an essential singularity to me. It looks like a simple pole. Why do you think it's essential? Did you try just computing the limit z->z0 (z-z0)*f(z)?
  4. Mar 25, 2013 #3
    Thank you for pointing that out, I must have been careless earlier. Using L'Hopital's rule on the indeterminate "0/0" form, I also now get that it is a simple pole. After that I used a result about Laurent series and residue about a pole of order m.

    Thank you.
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