@article{SF_1976__12_134189,
     author = {H.J. Shyr},
     title = {Every subsemigroup of a free semigroup with zero is not an {R-semigroup.}},
     journal = {Semigroup forum},
     pages = {380--382},
     year = {1976},
     volume = {12},
     zbl = {0338.20067},
     url = {http://geodesic.mathdoc.fr/item/SF_1976__12_134189/}
}