1. Orderings of ultrafilters;Blass,1970
2. Ultrafilter mappings and their Dedekind cuts;Blass;Trans. Am. Math. Soc.,1974
3. Finite preimages under the natural map from β(N×N) to βN×βN;Blass,2001
4. The Theory of Ultrafilters;Comfort,1974
5. Preimages of points under the natural map from β(N×N) to βN×βN;Hindman;Proc. Am. Math. Soc.,1973