This model for a 2D flow is adapted from the model Vector Field.nlogo by Uri Wilensky by changing the definition of the vectorfield only.

In this model, 2dflow-vdpol, adapted from 2dflow01, Uri's equations (expressed in the notation of Dynamics, the Geometry of Behavior):	

	x' = - y
	y' = + x

are changed into the Van der Pol equations following Hirsch-Smale 2nd edn, p. 270:

	x' = y - x^3 + mu * x
	y' = -x

with the control parameter mu in the interval [-1, 1]. For mu = 1 we have the usual Van der Pol system. As mu increases from -1, a Hopf bifurcation occurs at mu = 0. At this value, the attractive fixed point changes to a periodic attractor in a subtle bifurcation.