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Published online by Cambridge University Press: 09 April 2009
We answer in the negative (by considering an example) the problem posed in exercise 6 of § 2.3, page 62 [1], namely: If a D-class D of a semigroup S is a subsemigroup of S, then is D necessarily bisimple? For any semigroup T, we let LT, RT, HT, DT and JT denote Green's relations on T.