ČUPONA, Ǵorǵi; CELAKOSKI, Naum. FREE GROUPOIDS WITH xy2=xy. Matematichki Bilten, [S. l.], v. 21, n. 1, p. 5–16, 1997. Disponível em: https://bilten.smm.org.mk/index.php/bilten/article/view/1569. Acesso em: 14 aug. 2026.