New aspects of integrability of force-free Duffing-van der Pol   oscillator and related nonlinear systems
  In this letter we show that the force-free Duffing-van der Pol oscillator is completely integrable for a specific parametric choice. We derive a general solution for this parametric choice. Further, we describe a procedure to construct the transformation which removes the time dependent part from the first integral and provide the general solution by quadrature. The procedure is shown to have a wider applicability through additional examples. We also show that through our method one can deduce linearizing transformations in a simple and straightforward way and illustrate it with a specific example. 
V. K. Chandrasekar, M. Senthilvelan and M. Lakshmanan