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Re: Rearranging expressions in a user-defined form

  • To: mathgroup at smc.vnet.net
  • Subject: [mg94097] Re: Rearranging expressions in a user-defined form
  • From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
  • Date: Fri, 5 Dec 2008 05:27:52 -0500 (EST)
  • Organization: Uni Leipzig
  • References: <gh8hms$r38$1@smc.vnet.net>
  • Reply-to: kuska at informatik.uni-leipzig.de

Hi,

lets try the examples


expr = a*b + a*c + c*d + c*a - g*d - g*a

and

(expr /. a*x_ + a*2 c :> a*(x + c) + a*c) /.
  a*q_Plus + g_ :> a*q + Factor[g]

gives:
a (b + c) + (a + d) (c - g)

and

expr //. (q : a | d)*x_ + (q : a | d)*y_ :> q*(x + y)

gives:

d (c - g) + a (b + 2 c - g)

and

"third,fourth... form"

gives:

"third,fourth... form"

just as you wish.

Regards
   Jens

Alexei Boulbitch wrote:
> Dear MathGroup,
> 
> in symbolic transformations of mathematical expressions I often need to 
> group or transform differently different parts
> of these expression, just to make them more visible , or more easy to 
> analyze  or so.  Just to give a simplest  example, 
> assume that we have  an expression:
> 
> expr = ab + ac + cd + ca- gd - ga
> 
> and I need to have it once in the form like say,
> 
> expr=a (b + c) + (c - g) (d + a)
> 
> and another time in the form say,
> 
> expr=a (b + 2 c - g) + d (c - g)
> 
> or I will need to group it in some third, fourth ... form. You indeed 
> understand that I gave the example easily treatable without computer,
> while I have in mind much more complex transformations of real interest.
> 
> This evidently may be done by copy-pasting parts of the expression in 
> question and
> transforming then separately and then again copy-pasting and thus, 
> collecting the results together. That's exactly what I do in such a case.
> However, this may easily become a source of errors: such as copying 
> without a sign, or copying a part of the expression, rather than the 
> whole one and so on.
> Finally it considerably slows you down.
> 
> Could you think of tricks to instruct Mathematica to group only certain 
> terms and not others, and to take certain
> factors out of parentheses, while leave others, according to your choice?
> 
> Thank you, Alexei 
> 
> 
> 
> 
> 
> 


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