1. A. A. Andronov and E. A. Leontovich, “Dynamical systems with structural instability of the first degree in the plane”,Mat. Sb.,68, 328–372 (1965).
2. A. A. Andronov, E. A. Leontovich, I. I. Gordon, and A. G. Mayer,Qualitative Theory of Dynamical Systems of Second Order [in Russian], Nauka, Moscow (1966).
3. A. A. Andronov, E. A. Leontovich, I. I. Gordon, and A. G. Mayer,Bifurcation Theory of Dynamical Systems on the Plane [in Russian], Nauka, Moscow (1967).
4. A. A. Andronov and L. S. Pontryagin, “Structurally stable systems”,Dokl. Akad. Nauk SSSR,14, 247–250 (1937).
5. Dos Anjos, “Polynomial vector fields on a torus”,Bol. Soc. Bras. Mat.,17, No. 2, 1–22 (1986).