fminsearch returns complex numbers as a result

Hello,

I have system of ODE, which I've optimised using fminsearch. Unfortunately, the return number while used to obtain the numerical solution returns solution with imaginary part equal to zero. This is a problem as I want to use MatCont to do bifurcation. Is there a way to make sure that found parameters will return only real values? I've tried using command:

if any(~isreal(dxdt))

keyboard

end.

Unfortunately, while incorporating these few lines into my ODE function I'm getting only lines as the solution. Parameters which I've found with fminsearch produce harmonic behaviour of the solutions as expected. Is there any lines of code which I can add or other solver, which would fix this problem?

Thank you in advance.

[Torsten]

You must first find out why the ode solver produces complex numbers. This is usually the case if you use sqrt, ^ or log in the definition of your equations and the arguments for these three functions become negative.

[Malgorzata Wieteska]

Hello Torsten, Thank you for your answer. I've tried to use: options1 = odeset('Refine',13); options2 = odeset(options1,'NonNegative',13); Unfortunately, I'm getting error: Error using odeset (line 216); Unrecognized property name 'NonNegative'. Ia ny way to overcome this?

[Torsten]

You write that the returned solution has imaginary part, but it is equal to zero. Then why not convert the imaginary number into real by transferring only the real part to MatCont ?

Answer 1

[Jan]

If you are sure, that only the real part of the result matters, what about cropping the imaginary part?

finalY = real(y(end, :))

[Malgorzata Wieteska]

Thank you very much for your answer, I've tried this one, however, the solution of the ODE are in the form of complex numbers. So, it doesn't solve the problem. While using MatCont the solutions can't be a complex numbers.

[Jan]

So maybe your code contains a typo which causes the imaginary solution. Matlab does not do this without a reason. So insert a check in your code and let the debugger stop to examine, what's going on:

function dy = YourFcn(t, y)
...
if ~isreal(dy)
   disp('imaginary value')  % <-- set a breakpoint here
end
end

[Malgorzata Wieteska]

Thank you, I will do it