@article{SF_1977__15_134243,
     author = {R. M. Capocelli},
     title = {On some subsemigroups of a free semigroup.},
     journal = {Semigroup forum},
     pages = {125--136},
     year = {1977},
     volume = {15},
     zbl = {0393.20042},
     url = {http://geodesic.mathdoc.fr/item/SF_1977__15_134243/}
}