1. Semigroups admitting relative inverses;Clifford;Annals of Math.,1941
2. Embedding theorems for groups;Higman;J. London Math. Soc.,1949
3. Proper ordered inverse semigroups;Saîto;Pac. J. Math.,1965
4. Congruences on regular semigroups;Reilly;Pac. J. Math.,1967
5. F-inverse semigroups;McFadden;Proc. London Math. Soc.,1971