conversion from polynomial to symbolic

asked 2014-08-18 16:39:16 +0100

rws
1149 ●2 ●20 ●44 http://google.com/+Ral...

updated 2014-08-18 16:42:33 +0100

I want to convert a huge polynomial from ZZ[] into SR but with the following code I get an expression that is not expanded:

sage: R.<x> = ZZ[]
sage: var('n')
n

sage: p=16*x^5 - 20*x^3 + 5*x
sage: p.subs(x=n)
(4*(4*n^2 - 5)*n^2 + 5)*n
sage: SR(p)
(4*(4*x^2 - 5)*x^2 + 5)*x

Is there a way to get the result in expanded form without using expand()? Presumably the unnecessary grouping and subsequent expansion can take some time with huge polynomials, so I would like to prevent this from the start.

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answered 2014-08-18 23:29:12 +0100

tmonteil
27413 ●31 ●204 ●517 http://wiki.sagemath.o...

You can construct the symbolic polynomial from the list of coefficients of the algebraic one as follows:

sage: sum([b*n^a for (a,b) in enumerate(p)])
16*n^5 - 20*n^3 + 5*n

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That is indeed faster than SR(p).expand() so I'll accept the answer, many thanks.

rws ( 2014-08-19 07:48:08 +0100 )edit

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answered 2014-08-21 17:04:30 +0100

slelievre
17684 ●22 ●160 ●349 http://carva.org/samue...

If all you care about is the string you can do str(p).replace('x','n').

sage: R.<x> = ZZ[]
sage: p = 16*x^5 - 20*x^3 + 5*x
sage: p
16*x^5 - 20*x^3 + 5*x
sage: str(p).replace('x','n')
'16*n^5 - 20*n^3 + 5*n'

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