@article{GDZPPN001259075, title = {Subprincipal closed ideals in \ss N.}, journal = {Semigroup forum}, volume = {36}, year = {1987}, pages = {223-246}, zbl = {0659.22002}, url = {http://dml.mathdoc.fr/item/GDZPPN001259075} }
Hindman, N.; Davenport, D. Subprincipal closed ideals in ßN.. Semigroup forum, Tome 36 (1987) pp. 223-246. http://gdmltest.u-ga.fr/item/GDZPPN001259075/