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Rearranging expressions in a user-defined form

• To: mathgroup at smc.vnet.net
• Subject: [mg94083] Rearranging expressions in a user-defined form
• From: Alexei Boulbitch <Alexei.Boulbitch at iee.lu>
• Date: Thu, 4 Dec 2008 07:16:29 -0500 (EST)

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 






-- 
Alexei Boulbitch, Dr., Habil.
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• Follow-Ups:
  • Re: Rearranging expressions in a user-defined form
    • From: Andrzej Kozlowski <akoz@mimuw.edu.pl>