Re: Re: Ask Mathematica to sub for a long expression

*To*: mathgroup at smc.vnet.net*Subject*: [mg71929] Re: [mg71906] Re: Ask Mathematica to sub for a long expression*From*: Andrzej Kozlowski <akoz at mimuw.edu.pl>*Date*: Mon, 4 Dec 2006 06:39:09 -0500 (EST)*References*: <ekp4o4$2fq$1@smc.vnet.net><ekrkia$s72$1@smc.vnet.net> <200612031126.GAA08096@smc.vnet.net>

On 3 Dec 2006, at 20:26, aaronfude at gmail.com wrote: > Hi, > > Thanks for all the responses. It doesn't quite work for me, but maybe > with a little extra expertise it will. The problem is that that > pattern > is not always obvious w/o extra algebra. The basic problem is > summarized in the fact that the following doesn't work the way I'd > like > it to: > > -a-b/.a+b->c > > 2a+2b/.a+b->c > > Many thanks in advance! > > Aaron Fude > (Since I expect Daniel Lichtblau must have got tired of posting this again and again, I thought I might save him some labour. ) Take a look at this: http://forums.wolfram.com/mathgroup/archive/2006/Aug/msg00283.html For example: replacementFunction[-a-b,a+b-c,{a,b,c}] -c replacementFunction[2a+2b,a+b-c,{a,b,c}] 2 c where replacementFunction[expr_, rep_, vars_] := Module[ {num = Numerator[expr], den = Denominator[expr], poly = Numerator[Together[rep]]}, PolynomialReduce[num, poly, vars][[2]] / PolynomialReduce[den, poly, vars][[2]] ] Andrzej Kozlowski

**References**:**Re: Ask Mathematica to sub for a long expression***From:*aaronfude@gmail.com