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MathGroup Archive 2000

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Finding parts of Equations...

  • To: mathgroup at smc.vnet.net
  • Subject: [mg22031] Finding parts of Equations...
  • From: "Patrick E Crowe" <crow at gbis.com>
  • Date: Fri, 11 Feb 2000 02:38:13 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

I am relatively new to Mathematica; I read some of the earlier posts looking
for topics similar to what I ask now, but anything I found regarding pattern
matching was a little beyond my current Mathematica programming skill.  So
forgive me if I ask a question that has been recently broached and discussed
already.  I am trying to figure out how to find pieces of equations that are
hidden quite cleverly within a greater whole.  Specifically I am interested
in taking a given wholly symbolic function of one independant variable and
differentiating with respect to that variable, and then finding all
occurances of the original function within the derivative. Now I know that
Mathematica stores functions in a tree-like structure, and has built in
functions for searching, but I cannot seem to make these things work as I
wish them to.  So I would like to ask a few questions about Mathematica:

1)  Although I find that Mathematica creates the same tree structure for
slightly different representations of the same function, I have also found
that if I manipulate the function enough I can "fool" M into giving a
different structure for a function that I know to be identical to the first.
Is there some type of "unique tree representation" for each function, and if
so, how do I get there?  Will Simplify or FullSimplify do the trick?

2)  Some functions, when differentiated, contain multiple copies of the
original function.  Particularly rational functions involving exponentials.
However, it may be somewhat difficult to find these copies of the original
function because they may be raised to a power, multiplied by other terms,
or both.  They may require some amount of manipulation of the terms to
expose the copy.  I am trying to find a way to use Mathematica to find these
copies for me in lui of doing it by hand.  The standard search techniques
described in the Mathematica book do not seem to be working.  I need a more
robust searching tool that can take into account some of the associativities
involved in certain operations, as well as possible equalities, to find all
such occurances of a given set of terms within a greater whole.  The
techniques I have been using so far seem to be thwarted simply by powers or
products of searched for terms.  In other words, given some function F(t),
I do not seem to be able to produce code that recognises that a*F(t)^b
contains the function F(t), for any possible a and b (including other
functions of t).

Can anyone give me a few pointers, tell me of a good Mathematica programming
book that discusses searching in depth (but is accessible to someone who,
though having programming experience, is new to Mathematica), or direct me
to a web site that can help me with what I am trying to do?  I thank you for
your help in advance.




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