@article{SF_1980__19_134383,
     author = {R.M. Capocelli},
     title = {On optimal factorization of free semigroups into free subsemigroups.},
     journal = {Semigroup forum},
     pages = {199--212},
     year = {1980},
     volume = {19},
     zbl = {0428.20031},
     url = {http://geodesic.mathdoc.fr/item/SF_1980__19_134383/}
}