Complexity among the finitely generated subgroups of Thompson's group

Abstract

We demonstrate the existence of a family of finitely generated subgroups of Richard Thompson's group $F$ which is strictly well-ordered by the embeddability relation in type ${\epsilon }_0+1$. All except the maximum element of this family (which is $F$ itself) are elementary amenable groups. In fact we also obtain, for each $\alpha <{\epsilon }_0$, a finitely generated elementary amenable subgroup of $F$ whose EA-class is $\alpha +2$. These groups all have simple, explicit descriptions and can be viewed as a natural continuation of the progression which starts with $\mathbf{Z}+\mathbf{Z}$, $\mathbf{Z}\wr \mathbf{Z}$, and the Brin-Navas group $B$. We also give an example of a pair of finitely generated elementary amenable subgroups of $F$ with the property that neither is embeddable into the other.

47 pages. Substantially revised, with heaviest revisions in sections 8 and 9. Accepted for publication in Journal of Combinatorial Algebra