@article{SF_1984__30_134720,
     author = {H.J. Shyr},
     title = {A characterization of dense languages.},
     journal = {Semigroup forum},
     pages = {237--240},
     year = {1984},
     volume = {30},
     zbl = {0548.20050},
     url = {http://geodesic.mathdoc.fr/item/SF_1984__30_134720/}
}