What initial conditions should I use for my system of differential equations?

[latitude]

Hi, I've just started using Octave and I'd like to use it to solve and plot a system of differential equations, but I keep getting the same error which I don't understand. What have I done wrong in my code?

Outline:
Initial conditions
x1 = 1
x2 = 1.27 x 10^-6
x3 = 0

t = 0 - 140 days

Equations
dx1 = -0.5*x(1)*x(2)
dx2 = 0.5*x(1)*x(2) - 0.25*x(3)
dx3 = 0.25*x(2)

Code:
%function
function xdot = m (x, t)
xdot = zeros (3,1);
xdot(1) = -0.5*x(1)*x(2);
xdot(2) = 0.5*x(1)*x(2) - 0.25*x(3);
xdot(3) = 0.25*x(2);
endfunction

%initial conditions
x0 = [1;1.27e-06;0];
t = linspace (0,140);

%solve
y = lsode ("m",x0,t)

%plot
plot (y,t)


the error I get says
lsode: repeated convergence failures

Thanks!

 

[NateD]

The error you're getting is because your initial conditions are not compatible with the equations you defined in your code. Since your equation for dx2 has a negative coefficient, the initial condition for x2 must be greater than 0. Try changing the initial conditions to: x1 = 1x2 = 1x3 = 0