English

Relatively irreducible free subroups in Out($\mathbb{F}$)

Group Theory 2018-05-15 v3

Abstract

We prove that given a finite rank free group F\mathbb{F} of rank 3\geq 3 and two exponentially growing outer automorphisms ψ\psi and ϕ\phi with dual lamination pairs Λψ±\Lambda^\pm_\psi and Λϕ±\Lambda^\pm_\phi associated to them, and given a free factor system F\mathcal{F} with co-edge number 2\geq 2, ϕ,ψ\phi, \psi each preserving F\mathcal{F}, so that the pair (ϕ,Λϕ±),(ψ,Λψ±)(\phi, \Lambda^\pm_\phi), (\psi, \Lambda^\pm_\psi) is independent relative to F\mathcal{F}, then there \exists M1M\geq 1, such that for any integer m,nMm,n \geq M, the group ϕm,ψn\langle \phi^m, \psi^n \rangle is a free group of rank 2, all of whose non-trivial elements except perhaps the powers of ϕ,ψ\phi, \psi and their conjugates, are fully irreducible relative to F\mathcal{F} with a lamination pair which fills relative to F\mathcal{F}. In addition if both Λϕ±,Λψ±\Lambda^\pm_\phi, \Lambda^\pm_\psi are non-geometric then this lamination pair is also non-geometric. We also prove that the extension groups induced by such subgroups will be relatively hyperbolic under some natural conditions.

Keywords

Cite

@article{arxiv.1802.05705,
  title  = {Relatively irreducible free subroups in Out($\mathbb{F}$)},
  author = {Pritam Ghosh},
  journal= {arXiv preprint arXiv:1802.05705},
  year   = {2018}
}

Comments

Minor corrections, notation changes, updated references. Some proofs rewritten for better readability

R2 v1 2026-06-23T00:23:53.725Z