MathGroup Archive 2006

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Re: efficient term collection algorithm

Blake Laing wrote:
> Dear Math Group
> I wish to combine terms in a expression which share a denominator, such 
> as in the following simple case:
> In[1]:=
> a/i+b/j+c/i//.Plus[Times[A_.,Power[denom_,-1]],Times[B_.,Power[denom_,-1]]]:>
>       Factor[Plus[A,B]Power[denom,-1]]//InputForm
> Out[1]//InputForm=
> (a + c)/i + b/j
> The actual expression I am working with contains thousands of terms, and 
> a pairwise algorithm such as this is wholly inadequate. Will one of you 
> please suggest a more efficient way to combine each additive term in a 
> large expression with a shared denominator?

Hi Blake,

You could try the following:

expr = a/i + b/j + c/i;

a + c   b
----- + -
   i     j


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