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Math Help - Laurent Expansion of function

  1. #1
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    Laurent Expansion of function

    I want to find the Laurent expansion(i.e. the Laurent series) of \frac{z^2-1}{z^2+1}
    a) in the domain 0 < |z-i| < 1
    b) in the domain |z+i| > 2

    I know the solution in a) is \frac{z^2-1}{z^2+1}= \frac{i}{z-i} +\frac{1}{2} -\frac{1}{2}\sum_{n=1}^{\infty}(\frac{i}{2})^n (z-i)^n

    And in b) it is \frac{z^2-1}{z^2+1}= 1 + i\sum_{n=-2}^{-\infty}(2i)^{-n-1} (z+i)^n

    But why is that so?
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  2. #2
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    I will leave the details to you but this should get you started.

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

    Now if I factor out (z-i) this gives

    \frac{1}{(z-i)}\cdot \frac{1}{(z+i)}=\frac{1}{(z-i)}\cdot \frac{1}{(z-i+2i)}

    Now we factor a 2i out to get

    \frac{1}{(2i)(z-i)}\cdot \frac{1}{(1+\frac{z-i}{2i})}

    Now if we expand this in a geometric series we get

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

    Simplifying gives


    \frac{1}{(2i)(z-i)}\sum_{n=0}^{\infty} \left(\frac{i}{2}\right)^n(z-i)^n=\frac{1}{(4)}\sum_{n=0}^{\infty} \left(\frac{i}{2}\right)^{n-1}(z-i)^{n-1}

    Notice that

    \frac{z^2-1}{z^2+1}=\frac{z^2+1-2}{z^2+1}=1-2\left( \frac{1}{z^2+1}\right)

    From here just plug in and take two terms out of the series and reindex.

    The 2nd one is very similar but here is a hint

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

    Now expand this in a geometric series
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