@article{SF_1971__2_133813,
     author = {H.E. Scheiblich},
     title = {A characterization of a free elementary semigroup.},
     journal = {Semigroup forum},
     pages = {76--79},
     publisher = {mathdoc},
     volume = {2},
     year = {1971},
     zbl = {0226.20064},
     url = {http://geodesic.mathdoc.fr/item/SF_1971__2_133813/}
}