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],
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
. Furthermore, if G is the free group on A, then the “cut-down to generators”
which is a subsemigroup of
is the free inverse semigroup on A. Essentially the same result was given by Margolis and Pin [12].