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Re: Replacing an expression by an expression


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

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