JONSSON, B. Homogeneous Universal Relational Systems. MATHEMATICA SCANDINAVICA, [S. l.], v. 8, p. 137–142, 1960. DOI: 10.7146/math.scand.a-10601. Disponível em: https://www.mscand.dk/article/view/10601. Acesso em: 30 apr. 2024.