BACKELIN, J. On the number of semigroups of natural numbers. MATHEMATICA SCANDINAVICA, [S. l.], v. 66, p. 197–215, 1990. DOI: 10.7146/math.scand.a-12304. Disponível em: https://www.mscand.dk/article/view/12304. Acesso em: 14 apr. 2025.