1. Bruce, J.W., Giblin, P.J.-Generic curves and surfaces, J. London Math. Soc. (2) Vol. 24, 1981.
2. Darboux, G.-Sur la forme des lignes de courbure dans le voisinage d'un ombilic, Note VII; Leçons sur la théorie générale des surfaces, Vol. IV. Gauthier-Villars, 1896.
3. Gutiérrez, C., Sotomayor, J.-Structurally stable configurations of lines of principal curvature. To appear in Astérisque.
4. Gutiérrez, C.-Structural stability for flows on the torus with a cross-cap. Trans. AMS, Vol. 241, 1978.
5. Looijenga, E.-Structural stability of smooth families of C∞ functions, Thesis, Univ. of Amsterdam, 1974.