FERNÁNDEZ SÁNCHEZ, Percy. Acerca del problema de Poincaré. Pro Mathematica, [S. l.], v. 18, n. 35-36, p. 21–36, 2004. Disponível em: https://revistas.pucp.edu.pe/index.php/promathematica/article/view/10216. Acesso em: 16 dic. 2025.