@article{SF_1992__44_1_135133,
     author = {J.F. Berglund and N. Hindman},
     title = {Sums of idempotents   in {{\ss}N.}},
     journal = {Semigroup forum},
     pages = {107--111},
     publisher = {mathdoc},
     volume = {44},
     number = {1},
     year = {1992},
     zbl = {0759.22004},
     url = {http://geodesic.mathdoc.fr/item/SF_1992__44_1_135133/}
}