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