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

**References**:**Expression manipulation***From:*David <David.B.A.Epstein@googlemail.com>