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2. The Algebraic Theory of Semigroups;Clifford,1967
3. On the classification of a certain type of semigroup;Fantham,1960
4. On regular semigroups whose idempotents form a subsemigroup;Hall;Bull. Austral. Math. Soc.,1969
5. The maximum idempotent-separating congruence on an inverse semigroup;Howie,1964