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## Use of fsolve

**From**: |
Daniel Tourde |

**Subject**: |
Use of fsolve |

**Date**: |
Thu, 01 Oct 1998 18:08:46 +0200 |

Hello,
I would like to use the fsolve solution for solving non-linear equations
where sometimes, solutions or constant are very small:
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);
Where some of my nlcoef values are very small (between 1e-7 and 1e-10).
It happens that the solutions given by fsolve can be wrong due to
rounding and precision errors.
It is possible to increase the tolerance of fsolve by using the
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
I know the existence of some flags provided by fsolve:
Valid values and their meanings are:
-2: input error
-1: error encountered in user-supplied function
1: solution converged to requested tolerance
4: iteration limit exceeded
3: iteration is not making good progress
It happens during my computation that sometimes flag 3 or flag 4 are
raised. And of course, this is when I got wrong results.
Then my question is the following: How can I change the iteration limit
within fsolve. How can I improve the precision of Octave in its
globality ? In a word, what is the best way to get rid of flag 4 and
flag 3 ?
Thanks in advance
Daniel Tourde
--
*********************************************************************
Daniel TOURDE E-mail : address@hidden
The Aeronautical Research Institute of Sweden Tel : +46 8 634 13 44
P.O. Box 11021 S-161 11 BROMMA, Sweden Fax : +46 8 25 34 81
*********************************************************************

**Use of fsolve**,
*Daniel Tourde* **<=**