fsolve and complex solution?

Hello,

my test function is f=x^2+1 the slution is easy : x1=i orx2=-i (both are imaginary)

fsolve is not finding these values...why? fsolve returns 0 for a solution??

Thank you.

[Walter Roberson]

Please show your test code?

[John Miller]

x0 = 0; % Make a starting guess at the solution

options=optimset('Display','iter'); % Option to display output

[x,fval] = fsolve(@myfun,x0,options) % Call solver

with the funciotn:

function F = myfun(x)

F = x^2+1;

end

[Walter Roberson]

And you are not getting any warning message about it not being able to solve the problem?

I take it you overlooked this line in the documentation:

"fsolve only handles real variables. When x has complex variables, the variables must be split into real and imaginary parts."

[John Miller]

fsolve returns the following message:

No solution found.

fsolve stopped because the problem appears regular as measured by the gradient,

but the vector of function values is not near zero as measured by the

default value of the function tolerance.

<stopping criteria details>

x =0

fval =1

I should split x in real and imaginary? Like this:

F = (r+i*s)^2+1; ?

Answer 1

[Walter Roberson]

function F = myfun(ripair)
  x = ripair(1) + ripair(2)*1i;
  F = x^2+1;
 if imag(F) ~= 0; F = 10^5; end    %you need to emit finite real value from myfun
end

and make sure your x0 has a pair of values.