1. P. Alsholm, Existence of limit cycles for eneralized Liénard equations. J. Math. Anal. Appl. 171, 242–255 (1977)
2. L.A. Cherkas, Estimation of the number of limit cycles of autonomous systems. Differ. Uravn.13, 779–802 (1977); translation in Differ. Equ.13, 529–547 (1977)
3. F. Dumortier, J. Llibre, J.C. Artés, Qualitative Theory of Planar Differential Systems (Universitext, Springer, Berlin, 2006)
4. A. Gasull, H. Giacomini, M. Grau, Effective construction of Poincaré–Bendixson regions. J. Appl. Anal. Comp. 7, 1549–1569 (2017)
5. H. Giacomini, S. Neukirch, Number of limit cycles of the Liénard equation. Phys. Rev. E 56, 3809–3813 (1997)