GUCKENHEIMER, J. Principal Foliations of Surfaces near Ellipsoids. New Zealand Journal of Mathematics, [S. l.], v. 52, p. 361–379, 2021. DOI: 10.53733/126. Disponível em: https://nzjmath.org/index.php/NZJMATH/article/view/126. Acesso em: 20 may. 2022.