       Re: Replacing an expression by an expression

• To: mathgroup at smc.vnet.net
• Subject: [mg55646] Re: Replacing an expression by an expression
• From: Paul Abbott <paul at physics.uwa.edu.au>
• Date: Fri, 1 Apr 2005 05:36:17 -0500 (EST)
• Organization: The University of Western Australia
• References: <d2g6d7\$f7p\$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```In article <d2g6d7\$f7p\$1 at smc.vnet.net>, carlos at colorado.edu wrote:

> I have a 8 x 8 symbolic matrix B of complicated entries.
> Several partial expressions, however, can be simplified. Sample:
>
> rep= {(J12*x14-J11*y14)*(J12*(x32+x43)-J11*(y32+y43)) -> A0*(A1-A0)};
>
> The left expression appears verbatim in entry B[[1,1]], which is
>
> -(L21*(J12*x14-J11*y14)*(J12*(-x32-x43)+J11*(y32+y43)))/
> (16*A412*J*(J11^2+J12^2))

It is _not_ verbatim.

> But when I say B=B/.rep, nothing happens.

If you simplify B before applying the replacement rule it should work.
For example,

Simplify[-(L21*(J12*x14-J11*y14)*(J12*(-x32-x43)+J11*(y32+y43)))/
(16*A412*J*(J11^2+J12^2))] /. rep

works fine.

More generally, since your expressions appear to only involve
polynomials, PolynomialReduce is likely to be the best approach.

Cheers,
Paul

--
Paul Abbott                                   Phone: +61 8 6488 2734
School of Physics, M013                         Fax: +61 8 6488 1014
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```

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