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MathGroup Archive 2004

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Re: Reduce/Solve

  • To: mathgroup at smc.vnet.net
  • Subject: [mg49990] Re: Reduce/Solve
  • From: "Dana DeLouis" <delouis at bellsouth.net>
  • Date: Mon, 9 Aug 2004 04:29:32 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

Hello.  It is because of Mathematica's huge bug in variable names.  Reduce
often doesn't work because of this bug.  The first thing you need to try is
to change the variable name.  I would first start by changing the "r" to a
variable name that comes near the beginning of the alphabet, like "a" (at
least something before the "d" in the equation).

Taking your equation, and changing r to a:

eq = J == J0*(1 + r/d)*Exp[-(r/d)] /. r -> a

J == ((1 + a/d)*J0)/E^(a/d)

Now it should work:

tt = Reduce[eq, a]

d != 0 && J == 0 &&
   (J0 == 0 || a == -d) ||
  C[1] ? Integers &&
   J != 0 && J0 != 0 &&
   d != 0 && a ==
    -d - d*ProductLog[C[1],
       -(J/(E*J0))]

HTH
Sad, isn't it!

Dana


"Dr A.H. Harker" <a.harker at ucl.ac.uk> wrote in message
news:cetf3i$6i8$1 at smc.vnet.net...
> Dear All,
>          I tend to think of Reduce as a more powerful tool than Solve,
> yet with
>
> eq = J == J0 (1 + r/d) Exp[-r/d]
> ss = Solve[eq, r]
> tt = Reduce[eq, r]
>
>  I get useful output from Solve, but Reduce returns the expression
> unevaluated. What am I missing?  Mathematica 5.0.0.0 under Windows XP.
>
>        Tony
>
>  Dr A.H. Harker
>  Director of Postgraduate Studies
>  Deputy Head, Condensed Matter and Materials Physics Group
>  Department of Physics and Astronomy
>  University College London
>  Gower Street
>  LONDON
>  WC1E  6BT
>  (44)(0)207 679 3404
>  a.harker at ucl.ac.uk
>
>
>


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