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MathGroup Archive 1991

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Re: question about Solve

  • To: mathgroup at yoda.ncsa.uiuc.edu
  • Subject: Re: question about Solve
  • From: jacobson at cello.hpl.hp.com
  • Date: Wed, 09 Jan 91 08:11:52 PST

Paul Schatz writes:

> I want to solve some equations involving the matrix elements of
> u--insuring of course that u be orthogonal.  ... when I use Solve with
> the two conditions orth1 and orth4, I get four solutions. The third of
> these is correct. The others do not keep u orthogonal. Then I use
> Solve again. This time I add the additional condition orth2. It seems
> obvious to me that I should now get only the one correct solution.
> But in fact, Solve reports no solutions.

Solve frequently exhibits the problem of giving no solution.  When you
can get it to give you a list of solutions, one of which you know to be
right (the case at hand) you can use this trick.

	Select[Solve[...], (criterion/.#)&]

where "criterion" is an expresion that is true only for the correct
answer. 

  -- David Jacobson




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