@article{SF_1980__21_134450,
     author = {N. Hindman},
     title = {Sums equal to products in {{\ss}N.}},
     journal = {Semigroup forum},
     pages = {221--256},
     publisher = {mathdoc},
     volume = {21},
     year = {1980},
     zbl = {0448.22004},
     url = {http://geodesic.mathdoc.fr/item/SF_1980__21_134450/}
}