@article{SF_1977__15_134261,
     author = {W. Williams and C. Eberhart},
     title = {Elementary semigroups.},
     journal = {Semigroup forum},
     pages = {295--310},
     publisher = {mathdoc},
     volume = {15},
     year = {1977},
     zbl = {0403.20040},
     url = {http://geodesic.mathdoc.fr/item/SF_1977__15_134261/}
}