@article{10_1017_S0017089500007011,
     author = {Higgins, Peter M.},
     title = {Digraphs and the semigroup of all functions on a finite set},
     journal = {Glasgow mathematical journal},
     pages = {41--57},
     year = {1988},
     volume = {30},
     number = {1},
     doi = {10.1017/S0017089500007011},
     url = {http://geodesic.mathdoc.fr/articles/10.1017/S0017089500007011/}
}