@article{MS2_1968__22_166051,
     author = {E.M. Horadam},
     title = {Ramanujan's {Sum} and {Nagell's} {Totient} {Functio} for {Arithmetical} {Semi-Groups.}},
     journal = {Mathematica Scandinavica},
     pages = {269--281},
     publisher = {mathdoc},
     volume = {22},
     year = {1968},
     zbl = {0182.37601},
     url = {http://geodesic.mathdoc.fr/item/MS2_1968__22_166051/}
}