KRAICHUK , A. V. On a class of groups with invariant subgroups of the infinite index. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 40, n. 4, p. 440–445, 1988. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/9178. Acesso em: 6 dec. 2025.