Re: Beware of NSolve
- To: mathgroup at smc.vnet.net
- Subject: [mg50174] Re: Beware of NSolve
- From: "Kevin J. McCann" <kjm at KevinMcCann.com>
- Date: Thu, 19 Aug 2004 06:28:01 -0400 (EDT)
- References: <cfuq2n$64h$1@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
It is not clear to me why the extraneous roots are found. However, if
you set WorkingPrecision->40 in NSolve, you only get the two correct roots.
Kevin
Carlos Felippa wrote:
> 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.
>