ode45 Matlab system of differential equations

Hi,

I'm new to Matlab and I have been trying to solve/simulate values of my system of ordinary differential equations. Even when I copy and paste examples from the net Matlab tells me that my function is undefined. For example:

function ypsir =ypsir(t,y) a = .01; b = .1; ypsir(1) =-a*y(1)*y(2); ypsir(2) = a*y(1)*y(2)-b*y(2); ypsir(3) = b*y(2); ypsir = [ypsir(1) ypsir(2) ypsir(3)]';

Matlab will respond with

??? Input argument "y" is undefined.

Error in ==> Untitled at 4 ypsir(1) =-a*y(1)*y(2);

Can someone please explain what this means and how to fix it.

Thanks! :)

Answer 1

[Mischa Kim]

Leah, use something like

 function ODE_test()
 [t,Y] = ode45(@ypsir_ODE,[0 10],[1 1 1]);
 plot(t,Y)

 function ypsir = ypsir_ODE(t,y)
 a = .01; b = .1; 
 ypsir(1) =-a*y(1)*y(2); 
 ypsir(2) = a*y(1)*y(2)-b*y(2);
 ypsir(3) = b*y(2);
 ypsir = [ypsir(1) ypsir(2) ypsir(3)]';

• Re-name the ODE function name since you use it also as the return value; I used ypsir_ODE.
• Save the functions in one file, called ODE_test.m.

[Leah Powell]

thank you very much it worked :)