@article{SF_1974__6_133991,
     author = {Mati Kilp},
     title = {Commutative monoids all of whose principal ideals are projective.},
     journal = {Semigroup forum},
     pages = {334--339},
     publisher = {mathdoc},
     volume = {6},
     year = {1974},
     zbl = {0267.20053},
     url = {http://geodesic.mathdoc.fr/item/SF_1974__6_133991/}
}