Congruences on Orthodox Semigroups
John C. Meakin · Journal of the Australian Mathematical Society · 1971
A semigroup S is called regular if a ∈ aSa for every element a in S. The elementary properties of regular semigroups may be found in A. H. Clifford and G. B. Preston [1]. A semigroup S is called orthodox if S is regular and if the idempotents of S form a subsemigroup of S.