HORADAM, E. M. Ramanujan’s Sum and Nagell’s Totient Functio for Arithmetical Semi-Groups. Mathematica Scandinavica, Berkeley, CA, United States, v. 22, p. 269–281, 1968. DOI: 10.7146/math.scand.a-10891. Disponível em: https://journals.msp.org/mscand/article/view/2411. Acesso em: 25 jul. 2026.