Talk:Van der Pol oscillator
Missing reference is very important: Cartwright M. L. & Littlewood J. E.  On nonlinear differential equations of the second order, I: The equation y+k(1+y^2)y'+y = b\lambda k cos(�t+a), k large, Lond. Math. Soc. 20, 180-189. Their conclusion: chaos is a typical dynamical state.
The route to chaos: Van der Pol & Van der Mark -> Cartwright & Littlewood -> Levinson -> Smale -> horseshoe.
(reply) I added the above reference. I will revise the article later.