ČUPONA, Ǵorǵi; CELAKOSKI, Naum. FREE OBJECTS IN SOME VARIETIES OF GROUPOIDS. Matematichki Bilten, [S. l.], v. 20, n. 1, p. 5–16, 1996. Disponível em: https://bilten.smm.org.mk/index.php/bilten/article/view/1561. Acesso em: 14 aug. 2026.