An example involving a non-regular -class in a semigroup
THOMAS E. HALL · Journal of the Australian Mathematical Society · 1972
We answer in the negative (by considering an example) the problem posed in exercise 6 of § 2.3, page 62 [1], namely: If aD-classDof a semigroupSis a subsemigroup of S, then is D necessarily bisimple? For any semigroupT, we letLT,RT,HT,DTandJTdenote Green's relations onT.