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 The University of Western Australia (CRICOS Provider No 00126G) 35 Stirling Highway Crawley WA 6009 mailto:paul at physics.uwa.edu.au AUSTRALIA http://physics.uwa.edu.au/~paul