BACKELIN, Jörgen. On the number of semigroups of natural numbers. Mathematica Scandinavica, Berkeley, CA, United States, v. 66, p. 197–215, 1990. DOI: 10.7146/math.scand.a-12304. Disponível em: https://journals.msp.org/mscand/article/view/1319. Acesso em: 24 jul. 2026.