@article{SF_1975__11_134144,
     author = {Jonathan Leech},
     title = {The...-CATEGORY {OF} {A} {SEMIGROUP.}},
     journal = {Semigroup forum},
     pages = {283--296},
     year = {1975},
     volume = {11},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SF_1975__11_134144/}
}