SYSAK, Ya. P. On one question of B. Amberg. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 46, n. 4, p. 457–461, 1994. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/5748. Acesso em: 15 dec. 2025.