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MathGroup Archive 2004

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Beware of NSolve - nastier example

  • To: mathgroup at smc.vnet.net
  • Subject: [mg50207] Beware of NSolve - nastier example
  • From: carlos at colorado.edu (Carlos Felippa)
  • Date: Fri, 20 Aug 2004 04:58:03 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

This is a nasty one:

f=5/432-11/(27*Sqrt[70]*Sqrt[19-1890*x])+x/(2*Sqrt[38/35-(108+1/10000000)*x]);

Print[N[Solve[f==0,x]]]; (* OK, 3 roots #2,4,5 of a sextic poly *)
Print[NSolve[f,x,16]];   (* 6 roots, 3 wrong *)
Print[NSolve[f,x,21]];   (* 4 roots, 2 correct, 1 double wrong, 
        1 single wrong *)
Print[NSolve[f,x,24]];   (* 3 roots, 1 wrong *)
Print[NSolve[f,x,28]];   (* 3 roots, 1 wrong *)
Print[NSolve[f,x,32]];   (* 2 correct roots, missed 1 *)
Print[NSolve[f,x,64]];   (* 2 correct roots, missed 1 *)
Print[NSolve[f,x,128]];  (* 3 correct roots  *)

All 3 roots are single and well conditioned, no problem getting them
in single precision with NR.

Note: please reply to this newsgroup; DONT send mail to me directly
(there is a white list, if not in list  your msg >null)


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