LEADER, Solomon. On a Problem of Alfsen and Fenstad. MATHEMATICA SCANDINAVICA, [S. l.], v. 13, p. 44–46, 1963. DOI: 10.7146/math.scand.a-10687. Disponível em: https://www.mscand.dk/article/view/10687. Acesso em: 5 dec. 2025.