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MathGroup Archive 2004

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Re: Re: Beware of NSolve

  • To: mathgroup at smc.vnet.net
  • Subject: [mg50192] Re: [mg50176] Re: [mg50165] Beware of NSolve
  • From: "Janos D. Pinter" <jdpinter at hfx.eastlink.ca>
  • Date: Fri, 20 Aug 2004 04:57:24 -0400 (EDT)
  • References: <200408191028.GAA22850@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Optimization tools (NMInimize, MathOptimizer /Pro,...) can be used to 
sequentially find numerical solutions in pre-set boxes for the vars. I just 
checked this using MathOptimizer Pro that finds the correct solns in [-1, 
1], and reports no feas solns after that.

Janos D. Pinter


At 07:28 AM 8/19/2004, Bob Hanlon wrote:
>Same results with version 5.0.1 on a Mac; however, using higher precision
>corrected this:
>
>NSolve[f==0,x,WorkingPrecision->35]
>
>{
>   {x ->
>     -0.014126116704662366388244906316568330018795496579`3\
>4.69897000433602},
>   {x -> 0.00281659289513855686443538250704452049498597276\
>8`34.69897000433602}}
>
>
>Bob Hanlon
>
> >
> > From: carlos at colorado.edu (Carlos Felippa)
To: mathgroup at smc.vnet.net
> > Date: 2004/08/18 Wed AM 01:20:19 EDT
> > To: mathgroup at smc.vnet.net
> > Subject: [mg50192] [mg50176] [mg50165] Beware of NSolve
> >
> > Run v. 4.2 on Mac:
> >
> > f=5/432 - 11/(27*Sqrt[70]*Sqrt[19 - 1890*x]) + x/(2*Sqrt[38/35 -
>108*x]);
> >
> >  Solve[f==0,x]  returns 2 real roots:
> >
> > {{x -> (-171 - 25*Sqrt[105])/30240}, {x -> (-171 + 25*Sqrt[105])/30240}}
> >
> > NSolve[f==0,x]  returns 4 real roots:
> >
> > {{x -> -0.10481082961146104}, {x -> -0.014126116704662378},
> >  {x -> 0.002816592895138556}, {x -> 0.0003796126802330315}}
> >
> > Roots 1 and 4 are incorrect.  (Just plot f)
> >
> > Had a similar problem with a quartic 3 months ago.  This is a
> > simpler example.
> >
> >


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