@article{GDZPPN001261622, title = {Semigroup structures on \ss N.}, journal = {Semigroup forum}, volume = {44}, year = {1992}, pages = {238-244}, zbl = {0746.22002}, url = {http://dml.mathdoc.fr/item/GDZPPN001261622} }
Strauss, D. Semigroup structures on ßN.. Semigroup forum, Tome 44 (1992) pp. 238-244. http://gdmltest.u-ga.fr/item/GDZPPN001261622/