@article{SF_1978__16_134292,
     author = {N. Knox},
     title = {The inverse hull of the free semigroup on a set {X.}},
     journal = {Semigroup forum},
     pages = {345--354},
     publisher = {mathdoc},
     volume = {16},
     year = {1978},
     zbl = {0407.20047},
     url = {http://geodesic.mathdoc.fr/item/SF_1978__16_134292/}
}