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Re: Symbolic expand function doesn't always do anything
From: |
Benjamin Sapp |
Subject: |
Re: Symbolic expand function doesn't always do anything |
Date: |
13 Sep 2004 10:01:39 -0600 |
Hi,
Indeed, you are correct it is an octave integration peculiarity. You
see, octave interprets any thing that can be a number as a double
first. Then, when a double is used in a symbolic expression it is
converted to the exact form like in ginsh. By that time it's too late
because a 1 is already double approximation of a 1 rather than exactly a
1. You can perform your example from ginsh in octave by surrounding
each exact number with quotes and vpa(). For example on my computer:
octave:47> expand( (x-vpa("1"))^vpa("3"))
ans =
-1+x^3-3*x^2+3*x
It's a bit cumbersome but I don't think there's a better solution.
Good luck,
Ben.
On Mon, 2004-09-13 at 08:28, edA-qa mort-ora-y wrote:
> I'm using the OctaveForge integration of the GiNaC symbolic library and
> having some trouble using the "expand" function.
>
> I want is to perform the polynomial expansion:
>
> >x = sym( "x" )
> >expand( (x-1)^2 )
> ans = (x-1)^2
>
> I want to get
> ans = 1 + x^2 - 2*x
>
> If you do this form, it works:
> >expand( (x-1) * (x-1) )
> ans = 1.0+x^2-(2.0)*x
>
> Using ginsh with expand produces the desired results, so I'm assuming it
> is an Octave integration peculariaty:
>
> ginsh> expand( (x-1)^3 );
> -1-3*x^2+x^3+3*x
>
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- Symbolic expand function doesn't always do anything, edA-qa mort-ora-y, 2004/09/13
- Re: Symbolic expand function doesn't always do anything,
Benjamin Sapp <=
- Re: Symbolic expand function doesn't always do anything, Geraint Paul Bevan, 2004/09/13
- Re: Symbolic expand function doesn't always do anything, edA-qa mort-ora-y, 2004/09/13
- Re: Symbolic expand function doesn't always do anything, Jonathan Stickel, 2004/09/13
- Re: Symbolic expand function doesn't always do anything, edA-qa mort-ora-y, 2004/09/14
- CASs vs. octave numerics (was: Re: Symbolic expand function doesn't always do anything), Jonathan Stickel, 2004/09/14
- CASs vs. octave numerics (was: Re: Symbolic expand function doesn't always do anything), John W. Eaton, 2004/09/14
- Re: CASs vs. octave numerics (was: Re: Symbolic expand function doesn't always do anything), Joe Koski, 2004/09/14
- Re: Symbolic expand function doesn't always do anything, Paul Kienzle, 2004/09/13