@article{SF_1980__19_134377,
     author = {H.J. Shyr and D.C. Chu},
     title = {Monoids of languages admitting ring structures.},
     journal = {Semigroup forum},
     pages = {127--132},
     publisher = {mathdoc},
     volume = {19},
     year = {1980},
     zbl = {0435.20045},
     url = {http://geodesic.mathdoc.fr/item/SF_1980__19_134377/}
}