Every subsemigroup of a free semigroup with zero is not an R-semigroup.
Shyr, H.J.
Semigroup forum, Tome 12 (1976), p. 380-382 / Harvested from Göttinger Digitalisierungszentrum
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Publié le : 1976-01-01
EUDML-ID : urn:eudml:doc:134189 
@article{GDZPPN001251481,
     title = {Every subsemigroup of a free semigroup with zero is not an R-semigroup.},
     journal = {Semigroup forum},
     volume = {12},
     year = {1976},
     pages = {380-382},
     zbl = {0338.20067},
     url = {http://dml.mathdoc.fr/item/GDZPPN001251481}
}

Shyr, H.J. Every subsemigroup of a free semigroup with zero is not an R-semigroup.. Semigroup forum, Tome 12 (1976) pp. 380-382. http://gdmltest.u-ga.fr/item/GDZPPN001251481/