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Collecting terms in mathematica

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  1. Jun 3, 2014 #1
    I have an integrand with a handful of terms, and some of them have poles in the denominator of the form (x+c) (but not all). There are three poles in total, and I want to collect all the terms according to each pole individually (eg all the terms with (x-1) in the denominator, (x-5), etc.)

    How could I do this? I tried using collect just to see if mathematica would make any rearrangements at all, but I'm not having much luck with it.
     
  2. jcsd
  3. Jun 3, 2014 #2
    Without an example of what you have or what you have tried it is difficult to guess what to say.

    Is this anything like what you want to do?

    In[1]:= Collect[4/(x-g) + (a+b^2)/(x-f) + 2 q/(x-f) - 5/(x-f) - c/(x-g), {1/(x-f), 1/(x-g)}]

    Out[1]= (-5 + a + b^2 + 2 q)/(-f + x) + (4 - c)/(-g + x)
     
  4. Jun 4, 2014 #3
    Hi Bill!

    Sorry I should have been much more specific. Each term in my expression involves several products in their denominators, and of those terms, some of them have poles like (x-1) and (y-1) and sometimes both, like N/(x+1)(y+1)(x-1)... for example (this is a multi-dimensional integral). I'm a bit new to mathematica so my terminology or descriptions might be a bit weird.. but basically I have the expression in list form, and I found that, e.g.,

    Select[expr, MemberQ[#[[2]], (-1 + x)] &]

    (the [[2]] is just a detail regarding the arrangement of the list) picks out the terms I was looking for. Thanks!
     
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