SKOLEM, T. Some Remarks on the Triple Systems of Steiner. MATHEMATICA SCANDINAVICA, [S. l.], v. 6, p. 273–280, 1958. DOI: 10.7146/math.scand.a-10551. Disponível em: https://www.mscand.dk/article/view/10551. Acesso em: 17 jan. 2022.