Franck MICHEL on Sun, 23 Feb 2003 12:33:15 +0100 |
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Fast substitutions in Pari |
Hi, Assume u is a polynomial in x of degree n, and phi is a rational fraction in t. I would like to transform u to a truncated series in t, i.e: - replace x by phi in u; - and expand in t (to the degree n). I set seriesprecision to n and use: taylor(subst(u,x,phi),t); It is very simple; but is it the best way to do it? I have to do this task many times, with large polynomials, the speed is very important. I've tried to first expand phi in t; and to substitute the series in t into the polynomial in x. But it is slower, even if we add an O(t^p) to the series in t, with p chosen in function of x^k. Perhaps this kind of manipulation is already done by Pari with taylor(subst(u,x,phi),t) and there is no need to search to improve it (?) Thanks in advance for your advices. Franck