Qualitative analysis of phase--portrait for a class of planar vector   fields via the comparison method
  The phase--portrait of the second order differential equation: $$\ddot x+\sum_{l=0}^nf_l(x) \dot x^l=0 ,$$ is studied. Some results concerning existence, non--existence and uniqueness of limit cycles are presented. Among these, a generalization of the classical Massera uniqueness result is proved. 
Timoteo Carletti, Lilia Rosati, Gabriele Villari