Re: Simplifying complicated expressions

*To*: mathgroup at smc.vnet.net*Subject*: [mg110443] Re: Simplifying complicated expressions*From*: "Sjoerd C. de Vries" <sjoerd.c.devries at gmail.com>*Date*: Fri, 18 Jun 2010 01:25:52 -0400 (EDT)*References*: <hufu1o$eel$1@smc.vnet.net>

Mathematica does not replace the expression if I put a simple 2 in front of the first term, whereas most any human sees the simple reduction that's possible: In: 2 x^2 + y^3 - x^2 y^2 + Sqrt[z3 + y2] /. x^2 + y^3 - x^2 y^2 + Sqrt[z3 + y2] -> flxyz Out: 2 x^2 - x^2 y^2 + y^3 + Sqrt[y2 + z3] could be easily reduced to x^2 +flxyz Cheers -- Sjoerd On Jun 6, 12:42 pm, "David Park" <djmp... at comcast.net> wrote: > "The normal > /. and -> substitutions and patterns are not adequate for this." > > That sounds like a completely unfounded statement so why don't you > demonstrate it? > > David Park > djmp... at comcast.nethttp://home.comcast.net/~djmpark/ > > From: S. B. Gray [mailto:stev... at ROADRUNNER.COM] > > Suppose I have a long complex expression in which terms like > (x^2+y^3-x^2y^2+Sqrt[z3+y2]) (for a simple example) appear many times > along with various powers and the reciprocals of it, etc. To make the > expression comprehensible and to make the computation faster, I would > like to substitute say "f1xyz" for it everywhere it appears. The normal > /. and -> substitutions and patterns are not adequate for this. Of > course at evaluation time I want to compute f1xyz only once and not have > the final formula revert to the original variables. How do I prevent that? > > Also a welcome addition to Mathematica would be the ability to find these > repeated expressions automatically and put them in, because doing it > manually is very error-prone and slow. > > Tips will be appreciated! > > Steve Gray