4.17 One of the classic nonlinear differential equations is van der Pol's equation u" tu = Eu' (1 - u2) (E > 0). It first appeared in the study of nonlinear electric circuits. (i) Find an approximation of the limit cycle for van der Pol's equa- tion by substituting a perturbation series u = uo(t) teul (t) + . .., and computing uo and u1. Be sure that your approximation con- tains no unbounded terms. (ii) Compare (by graphing) the approximation that you computed in part a to a numerical approximation of the limit cycle for e = .5