JÓNSSON, Bjarni. Universal relational systems. Mathematica Scandinavica, Berkeley, CA, United States, v. 4, p. 193–208, 1956. DOI: 10.7146/math.scand.a-10468. Disponível em: https://journals.msp.org/mscand/article/view/2857. Acesso em: 25 jul. 2026.