There are many versions for the ``van der Pol’’ equation. Among them, there is this one investigated by Yoshisuke Ueda in 1965 in his Ph.D. thesis . It reads
This system is semi-conservative which means that there is a continuum of attractors, that is, many many different attractors co-exist in the state space . When parameter values are , , and combined with the initial conditions
there is a toroidal chaotic attractor (Fig. 1) which was related with the Curry-Yorke scenario in .
 Y. Ueda, Some problems in the theory of nonlinear oscillations, Ph.D. thesis, (1965) reprinted in The road to chaos, Aerial Press, 1992.
 O. Ménard, C. Letellier, J. Maquet, L. Le Sceller & G. Gouesbet, Analysis of a non synchronized sinusoidally driven dynamical system, International Journal of Bifurcation & Chaos, 10 (7), 1759-1772, 2000.
 C. Letellier, V. Messager & R. Gilmore, From quasi-periodicity to toroidal chaos : analogy between the Curry-Yorke map and the van der Pol system, Physical Review E, 77 (4), 046203, 2008.