DIRDAL, Gunnar. Congruence properties for a class of arithmetical functions. Mathematica Scandinavica, Berkeley, CA, United States, v. 38, p. 247–261, 1976. DOI: 10.7146/math.scand.a-11632. Disponível em: https://journals.msp.org/mscand/article/view/2015. Acesso em: 25 jul. 2026.