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How do I expand a function $f(z)$ in a particular region? For example, how would I expand $f(z)=(z^2-3z+2)^{-1}$ in the region $0<|z-1|<1.$? I believe this can be done by the binomial theorem. But how to arrange things to make the binomial expansion valid?

Please give me an idea for doing this.

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Is this a Mathematica question or a Math question? –  bobthechemist Mar 21 at 13:31

1 Answer 1

up vote 3 down vote accepted

In Mathematica:

Series[1/(z^2 - 3 z + 2), {z, 1, 10}, Assumptions -> {0 < Abs[z - 1 ]< 1}]

(or whatever the top-order term is that you want).

Mathematical method:

Step (i): expand the function into partial fractions -- in Mathematica:

Apart[1/(z^2 - 3 z + 2)]

One of the fractions you get is the the Laurent series term with negative power of $z - 1$.

Step (ii): Expand the other term using a binomial series.

Step (iii): Add the results of Steps (i) and (ii).

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okay thanks.What to do if the region is |z|<1 ? –  user3185198 Mar 21 at 17:27
    
For $|z| < 1$, you'll need to expand around $z = 0$, not $z = 1$. And from the code I provided, I think you can see how to express that in Mathematica. –  murray Mar 21 at 19:38
    
okay thanks.Can u explain the general case? –  user3185198 Mar 22 at 3:36

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