Regularising transformations for the (n,n+1) -Liénard equations

We discuss regularising transformations for the Liénard equations of the form y′′=F(z,y)y′+G(z,y), where F and G are polynomials of degrees n and n+1 respectively using the geometric approach. As a particular case we find a transformation for the Duffing–van der Pol equation which leads to the regularisation.