1. Carletti T, Rosati L, Villari G: Qualitative analysis of the phase portrait for a class of planar vector fields via the comparison method. Nonlinear Anal. 2007, 67(1):39–51. 10.1016/j.na.2007.01.019
2. Massera JL: Sur un Théoreme de G. Sansone sur l’equation de Liénard. Boll. Unione Mat. Ital. 1954, 9(3):367–369.
3. Sansone G: Soluzioni periodiche dell’equazione di Liénard. Calcolo del periodo. Rend. Semin. Mat. (Torino) 1951, 10: 155–171.
4. Sabatini M, Villari G: On the uniqueness of limit cycles for Liénard equation: the legacy of G. Sansone. Matematiche 2010, LXV(II):201–214.
5. Villari G: An improvement of Massera’s theorem for the existence and uniqueness of a periodic solution for the Liénard equation. Rend. Ist. Mat. Univ. Trieste 2012, 44: 187–195.