Pierre Joyal, Christiane Rousseau, “Saddle quantities and applications”, Journal of Differential Equations, 78:2 (1989), 374
G. Dangelmayr, J. Guckenheimer, “On a four parameter family of planar vector fields”, Arch. Rational Mech. Anal., 97:4 (1987), 321
J.P Francoise, C.C Pugh, “Keeping track of limit cycles”, Journal of Differential Equations, 65:2 (1986), 139
Ю. С. Ильяшенко, “Мемуар Дюлака “О предельных циклах” и смежные вопросы локальной теории дифференциальных уравнений”, УМН, 40:6(246) (1985), 41–78 ; Yu. S. Ilyashenko, “Dulac's memoir “On limit cycles” and related problems of the local theory of differential equations”, Russian Math. Surveys, 40:6 (1985), 1–49