@article{GDZPPN001253794, title = {On optimal factorization of free semigroups into free subsemigroups.}, journal = {Semigroup forum}, volume = {20}, year = {1980}, pages = {199-212}, zbl = {0428.20031}, url = {http://dml.mathdoc.fr/item/GDZPPN001253794} }
Capocelli, R.M. On optimal factorization of free semigroups into free subsemigroups.. Semigroup forum, Tome 20 (1980) pp. 199-212. http://gdmltest.u-ga.fr/item/GDZPPN001253794/