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algebraic substitution

asked 2014-02-06 11:38:29 +0200

czsan gravatar image

How can I imitate the algsubs function of Maple? A little example:

p = x^18

algsubs(x^2=x+1,p)

The reult in Maple is:

x^9+9x^8+36x^7+84x^6+126x^5+126x^4+84x^3+36x^2+9x+1.

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What is the mathematical meaning of this function ? If you want `x^2` to be equal to `x+1`, then the optimal result would be a polynomial of degree 1, namely `2584*x + 1597`.

tmonteil gravatar imagetmonteil ( 2014-02-06 14:57:54 +0200 )edit

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answered 2014-02-07 17:54:12 +0200

It seems that there are not the same function in sagemath but we can use " ratsubst " function of maxima

sage : maxima('p:x^18')
sage : maxima('ratsubst(a+1,x^2,p)')

how to use certain maxima functions (e.g. ratsubst)?

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answered 2014-02-06 12:28:02 +0200

ndomes gravatar image

updated 2014-02-06 12:58:02 +0200

kcrisman gravatar image
var('y')
P = x^18
eqn = x^2 == y + 1
soln = solve(eqn,x)
P = P.subs(x=soln[0].rhs())
P.expand().subs(y=x)
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It works only for even powers.

czsan gravatar imageczsan ( 2014-02-06 14:06:33 +0200 )edit

For example, if P = x^17 = x^16*x=(x^2)^4*x=(x+1)^4*x. But it is impossible with this method.

czsan gravatar imageczsan ( 2014-02-06 14:08:24 +0200 )edit

Out of curiosity, what do you want when you have `x^{17}`? Something with a square root? And if so, which square root?

kcrisman gravatar imagekcrisman ( 2014-02-06 14:30:50 +0200 )edit

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Asked: 2014-02-06 11:38:29 +0200

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Last updated: Feb 07 '14