text: E-semigroup
  
  In the area of mathematics known as semigroup theory, an E-semigroup is a semigroup in which the idempotents form a subsemigroup.
  
  Certain classes of E-semigroups have been studied long before the more general class, in particular, a regular semigroup that is also an E-semigroup is known as an orthodox semigroup.
  
  Weipoltshammer proved that the notion of weak inverse (the existence of which is one way to define E-inversive semigroups) can also be used to define/characterize E-semigroups as follows: a semigroup S is an E-semigroup if and only if, for all a and b ∈ S, W(ab) = W(b)W(a), where W(x) ≝ {x ∈ S | xax = x} is the set of weak inverses of x.
  
  References 
  
  Category:Semigroup theory
  Category:Algebraic structures