@article{SF_1982__25_134586,
     author = {K. Ahre},
     title = {On the closure of the bicyclic semigroup.},
     journal = {Semigroup forum},
     pages = {379--380},
     year = {1982},
     volume = {25},
     zbl = {0507.22004},
     url = {http://geodesic.mathdoc.fr/item/SF_1982__25_134586/}
}