@article{SF_1991__43_3_135097,
     author = {A.T. Lisan},
     title = {Ultrafilters on a discrete set with two binary operations.},
     journal = {Semigroup forum},
     pages = {77--82},
     publisher = {mathdoc},
     volume = {43},
     number = {3},
     year = {1991},
     zbl = {0723.22003},
     url = {http://geodesic.mathdoc.fr/item/SF_1991__43_3_135097/}
}