1. Analysis on Semigroups;Berglund,1989
2. The Theory of Ultrafilters;Comfort,1974
3. An action approach to semigroup compactifications;El-Mabhouh,1992
4. The semigroup βN and its applications to number theory;Hindman,1990
5. Free groups and semigroups in βN;Hindman;Semigroup Forum,1984