FIGUEIREDO, Djairo G. de. On some recent variational principles. Pro Mathematica, [S. l.], v. 3, n. 5-6, p. 53–64, 1989. Disponível em: https://revistas.pucp.edu.pe/index.php/promathematica/article/view/6062. Acesso em: 19 feb. 2026.