SELMER, Ernst S. On the postage stamp problem with the three stamp denominations. Mathematica Scandinavica, Berkeley, CA, United States, v. 47, p. 29–71, 1980. DOI: 10.7146/math.scand.a-11874. Disponível em: https://journals.msp.org/mscand/article/view/1772. Acesso em: 25 jul. 2026.