The intersection of free submonoids of a free monoid is free.
Tilson, B.
Semigroup forum, Tome 4 (1972), p. 345-350 / Harvested from Göttinger Digitalisierungszentrum
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Publié le : 1972-01-01
EUDML-ID : urn:eudml:doc:133915 
@article{GDZPPN001248510,
     title = {The intersection of free submonoids of a free monoid is free.},
     journal = {Semigroup forum},
     volume = {4},
     year = {1972},
     pages = {345-350},
     zbl = {0261.20060},
     url = {http://dml.mathdoc.fr/item/GDZPPN001248510}
}

Tilson, B. The intersection of free submonoids of a free monoid is free.. Semigroup forum, Tome 4 (1972) pp. 345-350. http://gdmltest.u-ga.fr/item/GDZPPN001248510/