PROTASOV, I. V. Morphisms of Ball’s Structures of Groups and Graphs. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 54, n. 6, p. 847–855, 2002. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/4121. Acesso em: 6 dec. 2025.