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MathGroup Archive 2004

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Re: Beware of NSolve

  • To: mathgroup at smc.vnet.net
  • Subject: [mg50176] Re: [mg50165] Beware of NSolve
  • From: Bob Hanlon <hanlonr at cox.net>
  • Date: Thu, 19 Aug 2004 06:28:05 -0400 (EDT)
  • Reply-to: hanlonr at cox.net
  • Sender: owner-wri-mathgroup at wolfram.com

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: [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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