[Scilab-users] solve complex ode

[Adrien Vogt-Schilb]

On 26/11/2012 20:33, Rodrigo Thomas wrote:
> Hello,
>
> I'm having a problem to solve complex ordinary differential equations 
> like (a simplified one):
>
> dx/dt = i*x.
>
> I tried to use:
>
> -->function xdot=f(x,t)
> -->xdot=%i*x
> -->endfunction
> -->x0=[1;0];
> -->t0=0;
> -->t=linspace(0,10,100);
> -->y=ode(x0,t0,t,f);
> -->plot(t,y)
>
> but Scilab give the plot of a constant line in x=1 and a exponential 
> term (real expoent). How can I solve it?
>
> hug,
> Thomas.
>
>

Hi Thomas,

Indeed, the result of this ODE is very different than the result of the 
naive one:

xx(:,1) = x0;
dt=0.1;
tt = 0:dt:10;
for i=1:size(t,"*")
xx(:,i+1) = xx(:,i)+dt*f(xx(:,i));
end


Now, the ODE help page seems to imply that ODE should be used with rela 
matrixes and functions

So, the solution seems to be to write real functions, something like

function xdot =f(x)
     im = x(1)
     re = x(2)

     dim = re
     dre = -im

     xdot = [dre; dim]
endfunction

hop this helps
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