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convergence of fsolve solution stepsize

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sarah on 1 Jun 2015
Commented: Star Strider on 1 Aug 2021
Any tips on convergence for fsolve function? My solution provides reasonable guesses based on inputs however it doesn't converge i.e. keeps giving error fsolve stopped because the relative size of the current step is less than the default value of the step size tolerance.
Is it possible to improve stepsize so it doesn't terminate?

Accepted Answer

Star Strider
Star Strider on 1 Jun 2015
You can set the step size tolerance and other performance characteristics to different values by creating an options structure with the optimoptions function. Pass the structure to your fsolve call as the third parameter.
For instance, your code might change to:
opts = optimoptions('fsolve', 'TolFun', 1E-8, 'TolX', 1E-8);
[B,fval] = fsolve(@(b) myfun(b,x,y), b0, opts);
and whatever other options you want to change, although the default value for 'FinDiffRelStep' is likely as low as it should go. Experiment with the others to get the result you want.

More Answers (1)

Alan Weiss
Alan Weiss on 2 Jun 2015
The exit message that you report does not indicate an error or a problem. fsolve is simply telling you why it stopped. See the documentation on exit flags and exit messages, and I suggest that you read the exit message part especially carefully.
Alan Weiss
MATLAB mathematical toolbox documentation
Star Strider
Star Strider on 1 Aug 2021
@Swarn Kalsi Most likely the Symbolic Math Toolbox.

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