@article{GDZPPN002287609,
title = {The Number of Functional Digraphs.},
journal = {Mathematische Annalen},
volume = {137},
year = {1959},
pages = {203-210},
zbl = {0087.38703},
url = {http://dml.mathdoc.fr/item/GDZPPN002287609}
}
HARARY, F. The Number of Functional Digraphs.. Mathematische Annalen, Tome 137 (1959) pp. 203-210. http://gdmltest.u-ga.fr/item/GDZPPN002287609/