1. V. S. Afraymovich and L.P. Shil’nikov, “On singular sets of Morse–Smale systems,” Tr. Mosk. Mat. Obs., 28, 181–214 (1973).
2. A. A. Andronov, E.A. Leontovich, I. I. Gordon, and A. G. Mayer, Qualitative Theory of Second-Order Dynamical Systems [in Russian], Nauka, Moscow (1966).
3. A. A. Andronov and L. S. Pontryagin, “Rough systems,” C. R. (Dokl.) Acad. Sci. URSS, n. Ser., 14, No. 5, 247–250 (1937).
4. D. V. Anosov, “Roughness of geodesic flows on compact Riemann manifolds of negative curvature,” Dokl. Akad. Nauk SSSR, 145, No. 4, 707–709 (1962).
5. D. V. Anosov, “Geodesic flows on close Riemann manifolds of negative curvature,” Trudy Mat. Inst. Steklov, 90 (1967).