VESTFRID, I. A. On the universal ultrametric space. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 46, n. 12, p. 1700–1706, 1994. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/5578. Acesso em: 5 dec. 2025.