1. Stability of Motion;Hahn,1967
2. On the global center of generalized Liénard equation and its application to stability problems;Hara;Funkcialaj Ekvacioj,1985
3. On a dynamical system in the Liénard plane. Necessary and sufficient conditions for the intersection with the vertical isocline and applications;Villari;Funkcialaj Ekvacioj,1990