1. The space of ultrafilters on N covered by nowhere dense sets;Balcar;Fund. Math.,1980
2. Spaces co-absolute with βN–N;Broverman;Topology Appl.,1981
3. The Theory of Ultrafilters;Comfort,1974
4. Parovičenko's characterization of βω − ω implies CH;van Douwen;Proc. AMS,1978
5. General Topology;Engelking,1977