@article{SF_1992__44_1_135147,
     author = {D. Strauss},
     title = {Semigroup structures on {{\ss}N.}},
     journal = {Semigroup forum},
     pages = {238--244},
     publisher = {mathdoc},
     volume = {44},
     number = {1},
     year = {1992},
     zbl = {0746.22002},
     url = {http://geodesic.mathdoc.fr/item/SF_1992__44_1_135147/}
}