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Multiplying large polynomials

  • To: mathgroup at
  • Subject: [mg14685] Multiplying large polynomials
  • From: Thomas Bell <tombell at>
  • Date: Sun, 8 Nov 1998 21:15:43 -0500
  • Sender: owner-wri-mathgroup at

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, and thanks in advance
for any suggestions.

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