Compressed words and automorphisms in fully residually free groups

The residual finiteness of (hyperbolic) automorphism-induced HNN-extensions

We classify finitely generated, residually finite automorphism-induced HNN-extensions in terms of the residual separability of a single associated subgroup. This classification provides a method to construct automorphism-induced HNN-extensions which are not residually finite. We prove that this method can never yield a "new" counter-example to Gromov's conjecture on the residual finiteness of hyperbolic groups.

Residual properties of automorphism groups of (relatively) hyperbolic groups

We show that Out(G) is residually finite if G is a one-ended group that is hyperbolic relative to virtually polycyclic subgroups. More generally, if G is one-ended and hyperbolic relative to proper residually finite subgroups, the group of outer automorphisms preserving the peripheral structure is residually finite. We also show that Out(G) is virtually p-residually finite for every prime p if G is one-ended and toral relatively hyperbolic, or infinitely-ended and virtually p-residually finite.

Nil-automorphisms of groups with residual properties

Following Plotkin we say that the automorphism $x$ of the group $G$ is a nil-automorphism if, for every $g\in G$, there exists $n=n(g)$ such that $[g,_n x]=1$. If the integer $n$ can be chosen independently of $g$, then $x$ is said to be unipotent. Nil and unipotent automorphisms can be regarded as a natural extension of the concept of Engel element, since a nil-automorphism $x$ is just a left-Engel element in $G < x >$. In this paper we consider nil-automorphisms of groups with residual properties namely locally-graded groups, residually-finite groups and profinite groups. The first result we…

Twisted conjugacy classes in residually finite groups

We prove for residually finite groups the following long standing conjecture: the number of twisted conjugacy classes of an automorphism of a finitely generated group is equal (if it is finite) to the number of finite dimensional irreducible unitary representations being invariant for the dual of this automorphism. Also, we prove that any finitely generated residually finite non-amenable group has the R-infinity property (any automorphism has infinitely many twisted conjugacy classes). This gives a lot of new examples and covers many known classes of such groups.

On the residual finiteness of outer automorphisms of relatively hyperbolic groups

We show that every virtually torsion-free subgroup of the outer automorphism group of a conjugacy separable relatively hyperbolic group is residually finite. As a direct consequence, we obtain that the outer automorphism group of a limit group is residually finite.

A lattice in a residually non-Desarguesian $\tilde A_2$-building

We build a building of type $\tilde A_2$ and a discrete group of automorphisms acting simply transitively on its set of vertices. The characteristic feature of this building is that its rank 2 residues are isomorphic to the Hughes projective plane of order 9, which is non-Desarguesian. This solves a problem asked by W. Kantor in 1986, as well as a question asked by J. Howie in 1989.

The Bass-Jiang group for automorphism-induced HNN-extensions

We use the Bass--Jiang group for automorphism-induced HNN-extensions to build a framework for the construction of tractable groups with pathological outer automorphism groups. We apply this framework to a strong form of a question of Bumagin--Wise on the outer automorphism groups of finitely presented, residually finite groups.

Subgroups and quotient groups of automorphism groups of RAAGs

We study subgroups and quotients of outer automorphism groups of right-angled Artin groups (RAAGs). We prove that for all RAAGS, the outer automorphism group is residually finite and, for a large class of RAAGs, it satisfies the Tits alternative. We also investigate which of these automorphism groups contain non-abelian solvable subgroups.

周期点与子移位自同构群的余有限性

Periodic points and residual finiteness of automorphism groups of subshifts

研究子移位的周期点密度与其自同构群余有限性之间的关系，证明在周期点不密集时，自同构群非余有限，并构造了一个非余有限的强不可约子移位例子。

Automorphism Groups of Graph Products of Buildings

We describe an underlying right angled building structure of any graph product of buildings. We describe the automorphism group of the graph product of buildings. We show that the notion of generalized graph product of a collection of groups, each with an action on a building, corresponds to the group of automorphisms generated via lifts. It follows that if each building is finite, then the generalized graph product is residually finite.

On a question of Bumagin and Wise

Motivated by a question of Bumagin and Wise, we construct a continuum of finitely generated, residually finite groups whose outer automorphism groups are pairwise non-isomorphic finitely generated, non-recursively-presentable groups. These are the first examples of such residually finite groups.

Normal automorphisms of relatively hyperbolic groups

An automorphism $α$ of a group $G$ is normal if it fixes every normal subgroup of $G$ setwise. We give an algebraic description of normal automorphisms of relatively hyperbolic groups. In particular, we prove that for any relatively hyperbolic group $G$, $Inn(G)$ has finite index in the subgroup $Aut_n(G)$ of normal automorphisms. If, in addition, $G$ is non-elementary and has no non-trivial finite normal subgroups, then $Aut_n(G)=Inn(G)$. As an application, we show that $Out(G)$ is residually finite for every finitely generated residually finite group $G$ with more than one end.

Compressed conjugacy and the word problem for outer automorphism groups of graph groups

It is shown that for graph groups (right-angled Artin groups) the conjugacy problem as well as a restricted version of the simultaneous conjugacy problem can be solved in polynomial time even if input words are represented in a compressed form. As a consequence it follows that the word problem for the outer automorphism group of a graph group can be solved in polynomial time.

Conjugacy p-separability of right-angled Artin groups and applications

We prove that every subnormal subgroup of p-power index in a right-angled Artin group is conjugacy p-separable. As an application, we prove that every right-angled Artin group is conjugacy separable in the class of torsion-free nilpotent groups. As another application, we prove that the outer automorphism group of a right-angled Artin group is virtually residually p-finite. We also prove that the Torelli group of a right-angled group is residually torsion-free nilpotent, hence residually p-finite and bi-orderable.

Twisted conjugacy in residually finite groups of finite Prüfer rank

Suppose, $G$ is a residually finite group of finite upper rank admitting an automorphism $φ$ with finite Reidemeister number $R(φ)$ (the number of $φ$-twisted conjugacy classes). We prove that such $G$ is soluble-by-finite (in other words, any residually finite group of finite upper rank, which is not soluble-by-finite, has the $R_\infty$ property). This reduction is the first step in the proof of the second main theorem of the paper: suppose, $G$ is a residually finite group of finite Prüfer rank and $φ$ is its automorphism with $R(φ)<\infty$; then $R(φ)$ is equal to the number of equivalence…

Virtually splitting the map from Aut(G) to Out(G)

We give an elementary criterion on a group G for the map from Aut(G) to Out(G) to split virtually. This criterion applies to many residually finite CAT(0) groups and hyperbolic groups, and in particular to all finitely generated Coxeter groups. As a consequence the outer automorphism group of any finitely generated Coxeter group is residually finite and virtually torsion-free.

Faithful actions of automorphisms on the space of orderings of a group

In this article we study the space of left- and bi-invariant orderings on a torsion-free nilpotent group $G$. We will show that generally the set of such orderings is equipped with a faithful action of the automorphism group of $G$. We prove a result which allows us to establish the same conclusion when $G$ is assumed to be merely residually torsion-free nilpotent. In particular, we obtain faithful actions of mapping class groups of surfaces. We will draw connections between the structure of orderings on residually torsion-free nilpotent, hyperbolic groups and their Gromov boundaries, and we s…

Small conjugacy classes in the automorphism groups of relatively free groups

Let F be an infinitely generated free group and R a fully invariant subgroup of F such that (a) R is contained in the commutator subgroup F' of F and (b) the quotient group F/R is residually torsion-free nilpotent. Then the automorphism group Aut(F/R') of the group F/R' is complete. In particular, the automorphism group of any infinitely generated free solvalbe group of derived length at least two is complete. This extends a result by Dyer and Formanek (1977) on finitely generated groups F_n/R' where F_n is a free group of finite rank n at least two and R a characteristic subgroup of F_n.

无限群上的逆模糊映射

Inverse ambiguous maps on infinite groups

该数学研究证明无限群必存在逆模糊函数，逆模糊自同构仅存在于阿贝尔群，明确有限生成阿贝尔群存在此类自同构的条件，还揭示局部有限群、剩余有限群的阿贝尔性与逆模糊自同构的关联。

Outer automorphism groups of graph products: subgroups and quotients

We show that the outer automorphism groups of graph products of finitely generated abelian groups satisfy the Tits alternative, are residually finite, their so-called Torelli subgroups are finitely generated, and they satisfy a dichotomy between being virtually nilpotent and containing a non-abelian free subgroup that is determined by a graphical condition on the underlying labelled graph. Graph products of finitely generated abelian groups simultaneously generalize right-angled Artin groups (RAAGs) and right-angled Coxter groups (RACGs), providing a common framework for studying these groups.…

Chirality in incidence geometry

Guided by the ideas of chirality in the abstract polytope theory, the present paper aims to extend the concept to a more general setting of incidence geometries. The purpose of this paper is to explore the more general framework of thin residually connected chiral geometries and also to take this opportunity to look at the regular case in a more detailed way. We give characterisations of automorphism groups of regular and chiral thin residually connected geometries in the same spirit as Coxeter groups.

Invariant group orderings and Galois conjugates

This paper investigates conditions under which a given automorphism of a residually torsion-free nilpotent group respects some ordering of the group. For free groups and surface groups, this has relevance to ordering the fundamental groups of three-dimensional manifolds which fibre over the circle.

On the residual finiteness of outer automorphisms of relatively hyperbolic groups

We show that every virtually torsion-free subgroup of the outer automorphism group of a conjugacy separable hyperbolic group is residually finite. As a result, we are able to prove that the group of outer automorphisms of every finitely generated Fuchsian group and of every free-by-finite group is resudually finite. We also generalize the main result for relatively hyperbolic groups.

Highly symmetric hypertopes

We study incidence geometries that are thin and residually connected. These geometries generalise abstract polytopes. In this generalised setting, guided by the ideas from the polytopes theory, we introduce the concept of chirality, a property of orderly asymmetry occurring frequently in nature as a natural phenomenon. The main result in this paper is that automorphism groups of regular and chiral thin residually connected geometries need to be $C$-groups in the regular case and $C^+$-groups in the chiral case.

稳定自同构群与 odometers 的全群

Stabilized automorphism groups and full groups of odometers

本文研究了 odometers 的稳定自同构群与拓扑全群的关系，证明了轨道等价与稳定自同构群同构的等价条件，并探讨了自由 $\mathbb{Z}^d$-odometers 的相关性质。

On the density of periodic configurations in strongly irreducible subshifts

Let $G$ be a residually finite group and let $A$ be a finite set. We prove that if $X \subset A^G$ is a strongly irreducible subshift of finite type containing a periodic configuration then periodic configurations are dense in $X$. The density of periodic configurations implies in particular that every injective endomorphism of $X$ is surjective and that the group of automorphisms of $X$ is residually finite. We also introduce a class of subshifts $X \subset A^\Z$, including all strongly irreducible subshifts and all irreducible sofic subshifts, in which periodic configurations are dense.

Entropy of automorphisms, homology and the intrinsic polynomial structure of nilpotent groups

We study the word length entropy of automorphisms of residually nilpotent groups, and how the entropy of such group automorphisms relates to the entropy of induced automorphisms on various nilpotent quotients. We show that much like the structure of a nilpotent group is dictated to a large degree by its abelianization, the entropy of an automorphism of a nilpotent group is dictated by its entropy on the abelianization. We give some applications to the study of pseudo-Anosov homeomorphisms of surfaces. In particular, we show that if $ψ$ is a non--homological pseudo-Anosov homeomorphism of a sur…

从望远镜到框架与单群

From telescopes to frames and simple groups

本文引入群的望远镜概念，模仿分支群构造得到若干 B-望远镜例子，并据此构造出 2-生成无限 amenable 单群、LEF 单群嵌入、有限单群直积中的 2-生成框架及 Grothendieck 对等应用。

多项式增长映射圆柱的同调增长

Homology growth of polynomially growing mapping tori

本文研究多项式增长自同构的映射圆柱的同调增长性质，证明其具有廉价重建性质并显示torsion同调增长在Farber序列下消失。

门格与稳定自同构群

Gate lattices and the stabilized automorphism group

该研究引入门格刻画剩余有限群上具最终填充性质的有限型子移位稳定自同构群的单整性，证明其整群为门格生成群的换位子群，且一维混合情形下稳定惰性自同构群是单群。

Test elements in pro-$p$ groups with applications in discrete groups

Let $G$ be a group. An element $g \in G$ is called a test element of $G$ if for every endomorphism $φ:G \to G$, $φ(g)=g$ implies that $φ$ is an automorphism. We prove that for a finitely generated profinite group $G$, $g \in G$ is a test element of $G$ if and only if it is not contained in a proper retract of $G$. Using this result we prove that an endomorphism of a free pro-$p$ group of finite rank which preserves an automorphic orbit of a non-trivial element must be an automorphism. We give numerous explicit examples of test elements in free pro-$p$ groups and Demushkin groups. By relating t…

Affine structures, wreath products and free affine actions on linear non-archimedean trees

Let $Λ$ be an ordered abelian group, $\mathrm{Aut}^+(Λ)$ the group of order-preserving automorphisms of $Λ$, $G$ a group and $α:G\to\mathrm{Aut}^+(Λ)$ a homomorphism. An $α$-affine action of $G$ on a $Λ$-tree $X$ is one that satisfies $d(gx,gy)=α_gd(x,y)$ ($x,y\in X$, $g\in G$). We consider classes of groups that admit a free, rigid, affine action in the case where $X=Λ$. Such groups form a much larger class than in the isometric case. We show in particular that unitriangular groups $\mathrm{UT}(n,\mathbb{R})$ and groups $T^*(n,\mathbb{R})$ of upper triangular matrices over $\mathbb{R}$ with p…

双侧移位的自同构与Higman--Thompson群 III：扩张

Automorphisms of the two-sided shift and the Higman--Thompson groups III: extensions

本文用Higman--Thompson群的外自同构子群解释移位动力系统自同构理论，实现惰性子群并证明自同构群包含多种重要群结构，同时扩展了相关自同构到外自同构群。

Large mapping tori of free group endomorphisms

We present an algorithm which, given any finite presentation of a group as input, will terminate with answer yes if and only if the group is large. We use this to prove that a mapping torus of a finitely generated free group automorphism is large if it contains the integers times the integers as a subgroup of infinite index. We then extend this result to mapping tori of finitely generated free group endomorphisms, as well as showing that such a group is large if it contains a Baumslag-Solitar group of infinite index and has a finite index subgroup with first Betti number at least 2. We also sh…

Reidemeister类、圈积与可解性

Reidemeister classes, wreath products and solvability

本文研究了限制圈积中的Reidemeister类，在自同构阶数和Reidemeister类数目有限的条件下证明了猜想TBFT_f，并证明了满足该条件的有限生成剩余有限群具有可解有限性（猜想R）。

纯虚拟孪生群的结构与自同构

Structure and automorphisms of pure virtual twin groups

本文研究纯虚拟孪生群的结构与自同构，证明其为具有平凡中心的不可约直角 Artin 群，给出精确展示及无限秩自由群迭代半直积分解，完整刻画其自同构群，并由此推出虚拟孪生群剩余有限及纯虚拟孪生群具有 $R_\infty$-性质。

Polynomial-time word problems

We find polynomial-time solutions to the word problem for free-by-cyclic groups, the word problem for automorphism groups of free groups, and the membership problem for the handlebody subgroup of the mapping class group. All of these results follow from observing that automorphisms of the free group strongly resemble straight line programs, which are widely studied in the theory of compressed data structures. In an effort to be self-contained we give a detailed exposition of the necessary results from computer science.

On endomorphisms of torsion-free hyperbolic groups

Let $H$ be a torsion-free $δ$-hyperbolic group with respect to a finite generating set $S$. Let $a_1,..., a_n$ and $a_{1*},..., a_{n*}$ be elements of $H$ such that $a_{i*}$ is conjugate to $a_i$ for each $i=1,..., n$. Then, there is a uniform conjugator if and only if $W(a_{1*},..., a_{n*})$ is conjugate to $W(a_1,..., a_n)$ for every word $W$ in $n$ variables and length up to a computable constant depending only on $δ$, $\sharp{S}$ and $\sum_{i=1}^n |a_i|$. As a corollary, we deduce that there exists a computable constant $\mathcal{C}=\mathcal{C}(δ, \sharp S)$ such that, for any endomorphism…

自由-循环群是共轭可分离的

Free-by-cyclic groups are conjugacy separable

本文证明了所有有限生成的自由-循环群都是共轭可分离的，并展示了外自同构群的残余有限性。