@article{SF_1975__11_134135,
     author = {T.E. Hall and C.J. Ash},
     title = {Invers {SEMIGROUPS} {ON} {GRAPHS.}},
     journal = {Semigroup forum},
     pages = {140--145},
     publisher = {mathdoc},
     volume = {11},
     year = {1975},
     zbl = {0334.20043},
     url = {http://geodesic.mathdoc.fr/item/SF_1975__11_134135/}
}