Published online by Cambridge University Press: 26 February 2010
In the terminology of Birget and Rhodes [3], an expansion is a functor F from the category of semigroups into some special category of semigroups such that there is a natural transformation η from F to the identity functor for which ηs is surjective for every semigroup S. The three expansions  introduced in [3] have proved to be of particular interest when applied to groups. In fact, as shown in [4],
 introduced in [3] have proved to be of particular interest when applied to groups. In fact, as shown in [4],  Ĝ(2) are isomorphic for any group G,
 Ĝ(2) are isomorphic for any group G,  is an E-unitary inverse monoid and the kernel of the homomorphism ηG is the minimum group congruence on
 is an E-unitary inverse monoid and the kernel of the homomorphism ηG is the minimum group congruence on  . Furthermore, if G is the free group on A, then the “cut-down to generators”
. Furthermore, if G is the free group on A, then the “cut-down to generators”  which is a subsemigroup of
 which is a subsemigroup of  is the free inverse semigroup on A. Essentially the same result was given by Margolis and Pin [12].
 is the free inverse semigroup on A. Essentially the same result was given by Margolis and Pin [12].