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MathGroup Archive 1996

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FindRoot problem

  • To: mathgroup at smc.vnet.net
  • Subject: [mg4812] FindRoot problem
  • From: "Arturas Acus" <acus at itpa.lt>
  • Date: Fri, 20 Sep 1996 01:12:37 -0400
  • Sender: owner-wri-mathgroup at wolfram.com

Dear All,

I came to the conclusion that FindRoot sometimes behaves a bit 
strange. Say I want to calculate:
x=x/.First[FindRoot[(x-Pi),{x,-4,4},AccuracyGoal->20, WorkingPrecision->30,
Compiled->False,MaxIterations->Infinity]

The result  of this is MachineNumber!!! It seems, problem is related 
with evaluation of Pi. And hint N[Pi,30]  do not help!!

Then I tried:

x=x/.First[FindRoot[x-Rationalize[N[Pi,30],0],{x,{-4,4}},
Compiled->False,AccuracyGoal->20,MaxIterations->Infinity,
WorkingPrecision->30]]

3.141592653589793238462643383279

Now it works correct. Does anybody had similar problem?
I tried two methods: secant and brent's. Both gives the same 
results. Can anybody explain this behaviour? This is important for my 
job.
                          Sincerely 
                                            Arturas Acus


                                     
Arturas Acus
Institute of Theoretical
Physics and Astronomy
Gostauto 12, 2600,Vilnius
Lithuania 


E-mail: acus at itpa.lt
   Fax: 370-2-225361
   Tel: 370-2-612906

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