%0 Journal Article
%A N. Hindman
%A J. Pym
%T Free groups and semigroups in  ßN.
%J Semigroup forum
%D 1984
%P 177-194
%V 30
%I mathdoc
%U http://geodesic.mathdoc.fr/item/SF_1984__30_134714/
%F SF_1984__30_134714