GÜL, Erdal. An application for the Gauss-Bonnet theorem. Pro Mathematica, [S. l.], v. 13, n. 25-26, p. 11–16, 1999. Disponível em: https://revistas.pucp.edu.pe/index.php/promathematica/article/view/8146. Acesso em: 28 dic. 2025.