Algebraic laminations for free products and arational trees

Abstract

This work is the first step towards a description of the Gromov boundary of the free factor graph of a free product, with applications to subgroup classification for outer automorphisms.

We extend the theory of algebraic laminations dual to trees, as developed by Coulbois, Hilion, Lustig and Reynolds, to the context of free products; this also gives us an opportunity to give a unified account of this theory. We first show that any $ℝ$–tree with dense orbits in the boundary of the corresponding outer space can be reconstructed as a quotient of the boundary of the group by its dual lamination. We then describe the dual lamination in terms of a band complex on compact $ℝ$–trees (generalizing Coulbois, Hilion and Lustig’s compact heart), and we analyze this band complex using versions of the Rips machine and of the Rauzy–Veech induction. An important output of the theory is that the above map from the boundary of the group to the $ℝ$–tree is 2-to-1 almost everywhere.

A key point for our intended application is a unique duality result for arational trees. It says that if two trees have a leaf in common in their dual laminations, and if one of the trees is arational and relatively free, then they are equivariantly homeomorphic.

This statement is an analogue of a result in the free group saying that if two trees are dual to a common current and one of the trees is free arational, then the two trees are equivariantly homeomorphic. However, we notice that in the setting of free products, the continuity of the pairing between trees and currents fails. For this reason, in all this paper, we work with laminations rather than with currents.