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## [Axiom-developer] [Axiom-mail] A newby question about eval and Expressio

 From: Martin Rubey Subject: [Axiom-developer] [Axiom-mail] A newby question about eval and Expression Date: Mon, 14 Nov 2005 02:46:30 -0600

```Changes http://page.axiom-developer.org/zope/mathaction/AxiomMail/diff
--

> Martin Rubey wrote:
>
> >Dear Doug,
> >
> >there are two issues in your code:
> >
> >
> >Doug Stewart writes:
> >
> >
> >>)clear all
> >>digits 20
> >>-- n:=x^3+a1*x^2+a2*x+a3 ::Polynomial Fraction Integer
> >>
> >>Q:=(3*a2-a1^2)/9
> >>R:=(9*a1*a2-27*a3-2*a1^3)/54
> >>S:=(R+(Q^3+R^2)^(1/2))^(1/3)
> >>T:=(R-(Q^3+R^2)^(1/2))^(1/3)
> >>x1:=S+T-a1/3
> >>
> >
> >note that x1 is *not* a polynomial. Not even a rational function.
> >
>
> But it is a constant ---- How can I tell axiom that it is a constant?

Maybe in a mathematical or physical sense, but not for Axiom, since it contains
variables.

You have to use the appropriate domain, for example EXPR INT itself. The way I
do these things usually is to ask Axiom for the "smallest" domain x1 is in, by
typing

x1

and looking at the type information.

You may also consider the domain POLY EXPR INT, which is, however, a dangerous
construction...

Martin

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