GRÄTZER, G.; WENZEL, G. H. On the Concept of Congruence Relation in Partial Algebras. MATHEMATICA SCANDINAVICA, [S. l.], v. 20, p. 275–280, 1967. DOI: 10.7146/math.scand.a-10837. Disponível em: https://tidsskrift.dk/math/article/view/10837. Acesso em: 13 aug. 2026.