@article{SF_1984__28_134661,
     author = {T. Nordahl},
     title = {Finitely generated left commutative semigroups are residually finite.},
     journal = {Semigroup forum},
     pages = {347--354},
     year = {1984},
     volume = {28},
     zbl = {0525.20046},
     url = {http://geodesic.mathdoc.fr/item/SF_1984__28_134661/}
}