Re: numerics

• To: mathgroup at smc.vnet.net
• Subject: [mg27270] Re: numerics
• From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
• Date: Sat, 17 Feb 2001 03:30:42 -0500 (EST)
• Organization: Universitaet Leipzig
• References: <96iqph\$d9j@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```Hi,

don't mix symbolic algorithms with floating point numbers.

The following will help

Reduce[Thread[newmat.{x1, x2, x3} == {0, 0, 0}] /.
x_Real :> Rationalize[x], {x1, x2, x3}] // N

How ever, the behaviour is a feature of Mathematica ;-)
Regards
Jens

Matt.Johnson at autolivasp.com wrote:
>
> mathematica gurus--
>
> Here's a frustrating problem, which seems to be simple.  I can calculate a
> matrix by hand and enter it into Solve or Reduce and get the correct
> relationships:
>
> In[108]:=
> mat = Partition[{-0.2, 0.1, 0, 0.1, -0.3, 0.1, 0.1, 0.2, -0.1}, 3];
> Reduce[mat.{x1, x2, x3} == {0, 0, 0}, {x1, x2, x3}]
> Out[109]=
> x1 == 0.5 x2 && x3 == 2.5 x2
>
> However, if I try to manipulate the matrix in Mathematica then solve, it doesn't work:
>
> In[110]:=
> mat1 = Partition[{0.8, 0.1, 0.1, 0.1, 0.7, 0.2, 0, 0.1, 0.9}, 3];
> imat = IdentityMatrix[Length[mat1]];
> newmat = Transpose[mat1] - imat;
> In[113]:=
> newmat == mat
> Out[113]=
> True
> In[114]:=
> newmat === mat
> Out[114]=
> False
> In[115]:=
> Reduce[newmat.{x1, x2, x3} == {0, 0, 0}, {x1, x2, x3}]
> Out[115]=
> x1 == 0. && x2 == 0. && x3 == 0.
>
> any input?
>
> -matt

```

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