HORADAM, E. M. Ramanujan’s Sum and Nagell’s Totient Functio for Arithmetical Semi-Groups. MATHEMATICA SCANDINAVICA, [S. l.], v. 22, p. 269–281, 1968. DOI: 10.7146/math.scand.a-10891. Disponível em: https://tidsskrift.dk/math/article/view/10891. Acesso em: 13 aug. 2026.