@article{SF_1984__30_134714,
     author = {N. Hindman and J. Pym},
     title = {Free groups and semigroups in  {{\ss}N.}},
     journal = {Semigroup forum},
     pages = {177--194},
     publisher = {mathdoc},
     volume = {30},
     year = {1984},
     zbl = {0536.22005},
     url = {http://geodesic.mathdoc.fr/item/SF_1984__30_134714/}
}