@article{SF_1978__16_134302,
     author = {M. Demlov\'a},
     title = {On groups of units in a special class of monoids.},
     journal = {Semigroup forum},
     pages = {443--454},
     publisher = {mathdoc},
     volume = {16},
     year = {1978},
     zbl = {0398.20061},
     url = {http://geodesic.mathdoc.fr/item/SF_1978__16_134302/}
}