@article{SF_1991__42_3_135073,
     author = {N. Hindman and J. Pym},
     title = {Closure of singly generated subsemigroups of  {S.}},
     journal = {Semigroup forum},
     pages = {147--154},
     publisher = {mathdoc},
     volume = {42},
     number = {3},
     year = {1991},
     zbl = {0721.22001},
     url = {http://geodesic.mathdoc.fr/item/SF_1991__42_3_135073/}
}