1. A general equation for relaxation oscillations;Levinson;Duke Math. J.,1942
2. Non-Linear Differential Equations;Sansone,1964
3. Qualitative theory of differential equations;Zhang;Transl. Math. Monographs,1992
4. Sopra l'equazione di A. Liénard delle oscillaxionidi rilassamento;Sansone;Ann. Mat. Pure Appl. (4),1949
5. Sur un théorème de G. Sansone sur l'equation de Liénard;Massera;Boll. Un. Mat. Ital. Ser. 3,,1954