@article{SF_1986__34_134827,
     author = {P.M. Higgins},
     title = {Dense subsets of some common classes of semigroups.},
     journal = {Semigroup forum},
     pages = {5--20},
     publisher = {mathdoc},
     volume = {34},
     year = {1986},
     zbl = {0601.20058},
     url = {http://geodesic.mathdoc.fr/item/SF_1986__34_134827/}
}