# Duffing equation ch1-duffing.ode 
# First, write the 2nd-order Duffing equation as a system
# of two first-order ODEs as follows:
dx/dt = y
dy/dt = -beta*y + x - x^3

# Set the default value of the parameter beta:
# Don't put spaces between the parameter name, the equal sign, and the parameter value!
par beta=1.0

# Set the default initial conditions:
init x=0.5  y=0.0

# Set the axes to plot a trajectory in the phase plane: 
@ xplot=x, yplot=y

# Set the stopping time, time step, and default viewing window
@ total=10,dt=.01,xlo=-2.0,xhi=2.0,ylo=-2.0,yhi=2.0
done