1. 1. A. A. Andronov and L. S. Pontrjagin, Systèmes grossiers, Dokl. Akad. Nauk vol. 14 (1937) pp. 247-250.
2. Dynamical systems with stable structures;DeBaggis, H. F.,1952
3. An estimate for the number of singular points of a dynamical system defined on a manifold;Èl′sgol′c, L. È.;Mat. Sbornik N.S.,1950
4. Self-intersections of immersed manifolds;Lashof, R. K.;J. Math. Mech.,1959