A new characterization of regular duo semigroups
Sándor Lajos · Proceedings of the Japan Academy Series A Mathematical Sciences · 1971
It is easy to see that the result of Theorem 2 remains true with quasi-ideal instead of bi-ideal.Theorem 3. A semigroup S is a vegular duo semigroup if and only if the condition ( 6 (Q QS)= Q (Q SQ) holds for every quasi-ideal Q of S.