solve complex ODE (like f'(x) = I*f(x)) numerically

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Hi!

i want so solve several complex valued (non-linear) ode's. I tried desolve_rk4. I got the error "Expecting a number when the initial state is replaced in the equations, but instead found: 0.5%i" in my equations, but also in the more simple one in the title. I can solve the equation f'(x) = If(x) symbolically, of course, but the equations i am interested in i want to solve numerically. So my question is (and i did not find an answer by reading the documentation or the google-results): Is it me how cannot pose the problem in a way that sage can solve it, or is it sage that cannot solve the problem.

Thank you for helping,

Max