[1] The notion of weak inverse is (as the name suggests) weaker than the notion of inverse used in a regular semigroup (which requires that axa=a).
The above definition of an E-inversive semigroup S is equivalent with any of the following:[1] This explains the name of the notion as the set of idempotents of a semigroup S is typically denoted by E(S).
[1] The concept of E-inversive semigroup was introduced by Gabriel Thierrin in 1955.
[2][3][4] Some authors use E-dense to refer only to E-inversive semigroups in which the idempotents commute.
[5] More generally, a subsemigroup T of S is said dense in S if, for all x ∈ S, there exists y ∈ S such that both xy ∈ T and yx ∈ T. A semigroup with zero is said to be an E*-dense semigroup if every element other than the zero has at least one non-zero weak inverse.