@article{CMUC_1978_19_2_a13,
     author = {Simon, Petr},
     title = {A somewhat surprising subspace of $\beta {\Bbb N}-{\Bbb N}$},
     journal = {Commentationes Mathematicae Universitatis Carolinae},
     pages = {383--388},
     year = {1978},
     volume = {19},
     number = {2},
     mrnumber = {0493972},
     zbl = {0378.54016},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/CMUC_1978_19_2_a13/}
}