1. V. I. Malykhin, “Extremally disconnected and nearly extremally disconnected groups,” Dokl. Akad. Nauk,220, No. 1, 27–30 (1975).
2. I. V. Protasov, “Filters and topologies on semigroups,” Mat. Studii. Pratsi L’vivsk. Mat. Tovarishch., No. 3, 15–28 (1994).
3. V. I. Malykhin, “Extremally disconnected topological groups,” Uspekhi Mat. Nauk,34, No. 6, 59–66 (1979).
4. N. Hindman and D. Strauss, “Nearly prime subsemigroup of βℕ, Semigroup Forum,51, No. 3, 299–318 (1995).
5. N. Hindman, “Ultrafilters and combinatorial number theory,” Lecture Notes in Math.,751, 49–184 (1979).