@article{SF_1987__36_134898,
     author = {N. Hindman and D. Davenport},
     title = {Subprincipal closed ideals in {{\ss}N.}},
     journal = {Semigroup forum},
     pages = {223--246},
     publisher = {mathdoc},
     volume = {36},
     year = {1987},
     zbl = {0659.22002},
     url = {http://geodesic.mathdoc.fr/item/SF_1987__36_134898/}
}