GRÄTZER, G.; WENZEL, G. H. On the Concept of Congruence Relation in Partial Algebras. Mathematica Scandinavica, Berkeley, CA, United States, v. 20, p. 275–280, 1967. DOI: 10.7146/math.scand.a-10837. Disponível em: https://journals.msp.org/mscand/article/view/2489. Acesso em: 25 jul. 2026.