1. American Math. Soc. Colloquium publications;G. Birkhoff,1948
2. Ergebnisse der Math., Neue Folge;R. H. Bruck,1958
3. Math. Surveys of the American Math. Soc.;A. H. Clifford,1961
4. HOWIE, J. M.: The maximum idempotent-separating congruence on an inverse semigroup. Proc. Edinburgh Math. Soc. (2), 14, 71–79 (1964).
5. KOCHIN, B. P.: The structure of inverse ideal-simple ω-semigroups. Vestnik Leningrad. Univ. 23, no. 7, 41–50 (1968).