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Precision of fsolve

From: John W. Eaton
Subject: Precision of fsolve
Date: Tue, 22 Sep 1998 07:47:36 -0500 (CDT)

On 22-Sep-1998, Daniel Tourde <address@hidden> wrote:

| I am using the function fsolve like this :
|   y(1) = 12*((x(3)*x(1))/2)/E - nlcoef(3);
|   y(2) = 12*((x(3)*x(2)+x(1)*x(4))/6)/E - nlcoef(4);
|   y(3) = 12*((P*x(3)/2+x(4)*x(2)+x(1)*x(5))/12)/E - nlcoef(5);
|   y(4) = 12*((P*x(4)/2+x(5)*x(2))/20)/E - nlcoef(6); 
|   y(5) = 12*((P*x(5))/60)/E - nlcoef(7);
| Some of my nlcoef values are very small (between 1e-7 and 1e-10) and it
| happens that the solutions send back by fsolve can be wrong due to
| rounding and precision errors. 
| My question is the following :
| How can I increase the precision of fsolve, how can I lower the
| admissible error ?

Try fsolve_options:

  octave:1> fsolve_options 

  *** fsolve_options:

  fsolve_options (KEYWORD, VALUE)

  Set or show options for fsolve.  Keywords may be abbreviated
  to the shortest match.

  Options for fsolve include:

    keyword                                  value
    -------                                  -----

    tolerance                                1.49012e-08

  octave:2> fsolve_options ("tol", 1e-12)
  octave:3> fsolve_options ("tol")
  ans =  1.0000e-12


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