@article{MAN_1959__138_160709,
     author = {F. HARARY},
     title = {The {Number} of {Functional} {Digraphs.}},
     journal = {Mathematische Annalen},
     pages = {203--210},
     publisher = {mathdoc},
     volume = {138},
     year = {1959},
     zbl = {0087.38703},
     url = {http://geodesic.mathdoc.fr/item/MAN_1959__138_160709/}
}