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Re: Expression manipulation

  • To: mathgroup at smc.vnet.net
  • Subject: [mg88731] Re: [mg88722] Expression manipulation
  • From: "W_Craig Carter" <ccarter at mit.edu>
  • Date: Thu, 15 May 2008 06:50:27 -0400 (EDT)
  • References: <200805141004.GAA09601@smc.vnet.net>

Hello David,
I believe there is a tutorial approximately titled "Simplifying
Expressions" in the documentation for 5.2.  Your items 2-5 can be done
automatically by methods described in that tutorial . The expression
you get may not be exactly the form you would choose when using pencil
and paper.

Item 1 requires a longer answer and a bit more finesse.  Replacements
of subexpressions can  be done with Replace and Rules. However, for
not-very-long expressions, you can copy paste the entire expression
and then select and over-write subexpressions.  I don't like this
second method so much because it is prone to mistakes---and a bad
habit to break---so I recommend looking at the Rule-Replace methods.
Craig



On Wed, May 14, 2008 at 6:04 AM, David <David.B.A.Epstein at googlemail.com> wrote:
> When trying to simplify an expression by hand, one carries out various
>  kinds of steps:
>  1. Replace a subexpression that occurs repeatedly by a single symbol.
>  2. Multiply numerator and denominator of some subexpression by the
>  same factor.
>  3. Cancel particular factors in numerator and denominator of some
>  subexpression.
>  4. Gather together two subexpressions that were added together, and
>  rewrite with a common denominator.
>  5. Remove common factors.
>
>  etc. etc. etc.
>
>  Using Part, one can of course access any particular subexpression. But
>  this is time-consuming and clumsy. I find that I need trial and error
>  to access the correct subexpression. Once I've accessed it, I often
>  have difficulty in persuading Mathematica to perform the desired
>  operation. And then I have trouble putting the subexpression back into
>  place. It's something like 20 times slower than working with pencil
>  and paper. HOWEVER pencil and paper calculations are more prone to
>  stupid arithmetic errors, particularly if the computation is a long
>  one.
>
>  I have been unable to find a convenient way of doing this in
>  Mathematica. I use version 5.2, but because of my University's site
>  license, I have access to more recent versions. Would it help to
>  change?
>
>  Can anyone point me to a tutorial where experts use Mathematica to do
>  a typical pencil and paper computation?
>
>  Thanks a lot. Please copy replies to my personal email address as I
>  don't look at the newsgroup very often.
>
>  David
>
>



-- 
W. Craig Carter


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