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MathGroup Archive 2002

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Re: Solve/Reduce and assumptions

  • To: mathgroup at smc.vnet.net
  • Subject: [mg32909] Re: [mg32887] Solve/Reduce and assumptions
  • From: BobHanlon at aol.com
  • Date: Tue, 19 Feb 2002 02:29:39 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

In a message dated 2/18/02 6:33:33 AM, hiranya at astro.princeton.edu writes:

>I have a large set of simultaneous multivariate algebraic equations which
>I am solving using Solve and Reduce. I get as the answer a list of all
>possible sets of solutions. However, only one of these is valid, because
>the equations contain a constant parameter R which should be positive.
>I
>cannot find any way of getting Solve and Reduce to recognize this fact.
>For example,
>
>Solve[{eqn1,eqn2,...eqnN,R!=0},vars] works, while
>
>Solve[{eqn1,eqn2,...eqnN,R>0},vars] is invalid.
>
>Applying
>
>Simplify[Result,R>0]
>
>to the full solution set does not work either.
>

You did not say what makes the results invalid.  I am guessing 
that you are looking for real solutions.  You can use Select, 
Cases, or DeleteCases.  For example, 

Select[
  Solve[R*x^3==8, x],
 
  Simplify[Element[x/.#,Reals], R>0]&]

{{x -> 2/R^(1/3)}}


Bob Hanlon
Chantilly, VA  USA


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