@article{SF_1991__42_3_135065,
     author = {N. Hindman},
     title = {Strongly summable ultrafilters on {N} and small maximal subgroups of {{\ss}N.}},
     journal = {Semigroup forum},
     pages = {63--76},
     publisher = {mathdoc},
     volume = {42},
     number = {3},
     year = {1991},
     zbl = {0716.22002},
     url = {http://geodesic.mathdoc.fr/item/SF_1991__42_3_135065/}
}