arXiv subjects
Publications and source records attributed to Oli Jones.
In this note, we exhibit a group which has a right-angled Artin group as a finite index subgroup, but no finite index normal subgroup isomorphic to any right-angled Artin group. This answers a recent question of Vankov.
We investigate fixed subgroups of automorphisms of generalised Baumslag-Solitar (GBS) groups. Our main results are for automorphisms leaving a Bass-Serre tree invariant, under the assumption that all edge stabilisers are strictly contained in the corresponding vertex stabilisers. We completely characterise which GBS groups admit such an automorphism with a fixed subgroup which is not finitely-generated. In doing so, we provide an infinite family of examples of non-finitely generated fixed subgroups in GBS groups. Dropping the above assumptions, we show that all finite order automorphisms of GBS groups have finitely generated fixed subgroups. Furthermore, we show that when the GBS graph is a tree, all automorphisms have finitely generated fixed subgroups.
We reduce a strong version of the twist conjecture for Artin groups to Artin groups whose defining graphs have no separating vertices. This produces new examples of Artin groups satisfying the conjecture, and sheds more light on the isomorphism problem for Artin groups. Along the way we also prove a combination result for the ribbon property for vertices.
We prove that an Artin group splits over infinite cyclic subgroups if and only if its defining graph has a separating vertex, and explicitly construct a JSJ decomposition over infinite cyclic subgroups for all Artin groups. We then use these facts to show that, if two Artin groups are isomorphic, then they have the same set of parabolics supported on "big chunks", that is, maximal subgraphs without separating vertices. We also deduce acylindrical hyperbolicity for the automorphism groups of many Artin groups, partially answering a question of Genevois in the case of Artin groups. As a consequence, we produce new families of Artin groups with the R-infinity property.
Given a connected large-type Artin group $A_\Gamma$, we introduce a deformation space $\mathcal{D}$. If $\Gamma$ is triangle-free, or has all labels at least 6, we show that this space is canonical, in that it depends only on the isomorphism type of $A_\Gamma$, and admits an $\Out(A_\Gamma)$-action. Using this action we conclude that $\Out(A_\Gamma)$ is of type VF, which implies $\Out(A_\Gamma)$ finitely presentable. We emphasise that our proof can handle cases where $\Gamma$ has separating vertices, which were previously problematic. In fact, our proof works for all connected large-type Artin groups satisfying the technical condition of having rigid chunks. We conjecture that all connected large-type Artin groups have rigid chunks, and therefore outer automorphism groups of type VF.
We study fixed subgroups of automorphisms of any large-type Artin group $A_{\Gamma}$. We define a natural subgroup $\mathrm{Aut}_\Gamma(A_\Gamma)$ of $\mathrm{Aut}(A_{\Gamma})$, and for every $\gamma \in \mathrm{Aut}_\Gamma(A_\Gamma)$ we find the isomorphism type of $\mathrm{Fix}(\gamma)$ and a generating set for a finite index subgroup. We show that $\mathrm{Fix}(\gamma)$ is a finitely generated Artin group, with a uniform bound on the rank in terms of the number of vertices of $\Gamma$. Finally, we provide a natural geometric characterisation of the subgroup $\mathrm{Aut}_\Gamma(A_\Gamma)$, which informally is the maximal subgroup of $\mathrm{Aut}(A_\Gamma)$ leaving the Deligne complex of $A_{\Gamma}$ invariant.
We completely classify fixed point subgroups in Torus Knot Groups, that is groups of the form $G_{p,q} = \langle x , y | x^p = y^q \rangle$. We not only give the isomorphism type, but also the explicit generators for the fixed point subgroup of each automorphism of $G_{p,q}$. Our main tool is an action of $Aut(G_{p,q})$ on the Bass-Serre tree of $G_{p,q}$ which is compatible with the original action, in the sense that it extends the original action of $G_{p,q}$ on its Bass-Serre tree.