1. Algebra in the Stone–Čech Compactification: Theory and Applications;Hindman,1998
2. Free groups and semigroups in βN;Hindman;Semigroup Forum,1984
3. Extremally disconnected and nearly extremally disconnected topological groups;Malyhin;Dokl. Nauk SSSR,1975
4. Topological groups with finite semigroups of ultrafilters;Zelenyuk;Matematychni Studii,1996