@article{SF_1974__6_133957,
     author = {T.E. Nordahl},
     title = {Commutative semigroups whose proper subsemigroups are projective.},
     journal = {Semigroup forum},
     pages = {35--41},
     publisher = {mathdoc},
     volume = {6},
     year = {1974},
     zbl = {0275.20105},
     url = {http://geodesic.mathdoc.fr/item/SF_1974__6_133957/}
}