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Re: Multiplying large polynomials


Thomas Bell wrote:
> 
> I'm trying to multiply two huge polynomials, and memory is a major
> concern.  I want to truncate the resulting polynomial to a specified
> power (N) in one of the variables (h), and I was wondering if it was
> possible to tell Mathematica to not bother multiplying two terms if the
> resulting power of h was greater than N.  This is, of course, in the
> hope that this "automatic truncation" would save time and memory.  For
> example, if
> 
> poly1 = a h^2 + b;
> poly2 = d h + e;
> N = 2;
> 
> then I would like to result to be
> 
> result = a e h^2 + b d h + b e
> 
> Instead, I have to write
> 
> result = Expand[poly1 poly2]/.h^3 -> 0;
> 
> which forces Mathematica to create the enormous product before
> truncating.  Please cc to tombell at stanford.edu, and thanks in advance
> for any suggestions.

-- 

In[1]:=  poly1=a h^2+b + O[h]^3;

In[2]:= poly2 = d h+e + O[h]^3;

In[3]:=  poly1 poly2 // Normal //InputForm

Out[3]//InputForm= b*e + b*d*h + a*e*h^2


Rolf Mertig

Mertig Research & Consulting  
Mathematica training and programming Development and distribution of
FeynCalc  Amsterdam, The Netherlands
http://www.mertig.com


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