Twisted Conjugacy Classes in Abelian Extensions of Certain Linear Groups

Abstract

Given an automorphism $ϕ:Γ\to Γ$, one has an action of $Γ$ on itself by $ϕ$-twisted conjugacy, namely, $g.x=gxϕ(g^{-1})$. The orbits of this action are called $ϕ$-twisted conjugacy classes. One says that $Γ$ has the $R_\infty$-property if there are infinitely many $ϕ$-twisted conjugacy classes for every automorphism $ϕ$ of $Γ$. In this paper we show that SL$(n,\mathbb{Z})$ and its congruence subgroups have the $R_\infty$-property. Further we show that any (countable) abelian extension of $Γ$ has the $R_\infty$-property where $Γ$ is a torsion free non-elementary hyperbolic group, or SL$(n,\mathbb{Z})$, Sp$(2n,\mathbb{Z})$ or a principal congruence subgroup of SL$(n,\mathbb{Z})$ or the fundamental group of a complete Riemannian manifold of constant negative curvature.