On the uniqueness of limit cycles for Li\'enard equation: the legacy of   G. Sansone
  We give an account of the results about limit cycle's uniqueness for Li\'enard equations, from Levinson-Smith's one to the most recent ones. We present a new uniqueness theorem in the line of Sansone-Massera's geometrical approach. 
M. Sabatini, G. Villari
math.CA math.DS