@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/