Global lyapunov exponent

[hn1]

How can I find global lyapunov exponent for a nonlinear system with some input pulse dependent on the system parameters like:

D(x)=c*x+d*y+f(x,y)+g(t) (g=pulse, f={1 if x>y and 0 otherwise})
D=a*x+b*y

how can I find the lyapunov exponent for a 2D system/

thanks
hn1