Published online by Cambridge University Press: 09 April 2009
It is well-known that, in any semigroup S, all the left and right Schützenberger groups of the ℋ-class contained in a fixed D class D of S are isomorphic to one group. We prove a sharper result: that, up to actionpreserving isomorphisms, all these Schützenberger groups and their classical isomorphisms are but one group and its identity isomorphism; thus, one group is essentially sufficient to describe not only all the Schützenberger groups of ℋ-classes in D, but their action on these ℋ-classes and classical isomorphisms as well.