Explicit computation of the index of a positive outer automorphism of the free group
Yann Jullian
给出了一个求自由群的正外自同构的指数的算法，并证明了该算法在有限时间内存在。
We give an algorithm for finding the index of a positive outer automorphism of the free group, and prove the algorithm exits in a finite time.
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Boundary actions of outer automorphism groups of Thompson-like groups
本文研究了索恩-汤普森群的外自同构群的边界作用，并证明这些群是C*-简单。
Fractal trees for irreducible automorphisms of free groups
本原代换的吸引子移位的自相似结构被推广到自由群的不可约外自同构的外空间边界上的排斥树的极限集上.因此，这个排斥树是自相似的（在图定向构造的意义上）。计算了它的豪斯多夫维数。这揭示了自由群的不可约外自同构在外空间边界上的吸引树的分形性质。
Using abelian coverings of Salvetti complexes, embeddings of outer automorphism groups of right-angled Artin groups (RAAGs) into outer automorphism groups of their particular characteristic subgroups are constructed. Virtual embeddings of outer automorphism groups of finitely generated groups having the unique root property into outer automorphism groups of their particular subgroups are also given. These results provide us with rich examples of (virtual) embeddings between outer automorphism groups of RAAGs. Finite subgroups of pure (outer) automorphism groups of RAAGs are also investigated. …
Generators for the Pure Symmetric Outer Automorphisms of a Free Product
本文通过构造自由积外空间的良好子复形，几何地证明了给定分裂的纯对称外自同构可由因子外自同构与相对Whitehead外自同构生成。
On the outer automorphism groups of free groups
该研究证明了秩可数无穷的自由群的外自同构群具有完备性，丰富了自由群外自同构群相关的代数理论结论。
$p$-Groups for which each outer $p$-automorphism centralizes only $p$ elements
群的自同构称为外自同构，如果它不是内自同构。设$G$是有限$p$ -群.则对于$G$的每个外$p$ -自同构$ϕ$，子群$C_G(ϕ)=\{x\in G \;|\; x^ϕ=x\}$有阶$p$当且仅当$G$至多有阶$p^2$。
Explicit examples of equivalence relations and factors with prescribed fundamental group and outer automorphism group
本文给出了具有给定基本群和外自同构群的II $_1$因子和II $_1$等价关系的一些显式构造。我们构造了与$\IRpos$不同的具有不可数基本群的因子和关系。事实上，给定任何II $_1$等价关系，我们构造具有相同基本群的II $_1$因子。给定任何局部紧幺模第二可数群$G$，我们构造了一个外自同构群为$G$的II $_1$等价关系$\RelR$.同样的构造没有给出以$G$为外自同构群的II型$_1$因子，但是当$G$是紧群或如果是$G=\SL^{\pm}_n\IR=\{g\in\GL_n\IR\mid \det(g)=\pm1\}$，则我们仍然找到其外自同构群是$G$的II型$_1$因子.
In this paper, we study some properties of the outer automorphism group of free Burnside groups of large odd exponent. In particular, we prove that it contains free and free abelian subgroups.
Complete outer automorphism groups of free nilpotent groups
本研究证明了2类无限生成自由幂零群的外自同构群具有完全性，推进了无限生成自由幂零群外自同构群的结构性质研究。
Bumagin-Wise posed the question of whether every countable group can be realised as the outer automorphism group of a finitely generated, residually finite group. We give a partial answer to this problem for recursively presentable groups.
Parageometric outer automorphisms of free groups
本文研究有限秩自由群F_r的全不可约外自同构φ，它们是“parageometric”的，即φ在外空间边界上的吸引不动点是关于F_r作用的几何R-树，但φ本身不是几何外自同构，因为它不是曲面的同胚。我们的主要结果表明，φ的展开因子严格大于φ的逆的展开因子。作为推论（由Guirardel独立证明），一个parageometric外自同构的逆既不是几何的也不是parageometric的，一个完全不可约的外自同构是几何的当且仅当它在外空间边界上的吸引和排斥不动点是几何R-树。
Characterizing inner automorphisms and realizing outer automorphisms
本文用纯群论方法给出有限群自同构的两个定理的初等证明：内自同构的扩张刻画，以及任意有限群可实现为某有限群的外自同构群。
We introduce a homology theory for subspace arrangements, and use it to extract a new system of numerical invariants from the Bieri-Neumann-Strebel invariant of a group. We use these to characterize when the set of basis conjugating outer automorphisms (a.k.a. the pure symmetric outer automorphism group) of a right-angled Artin group is itself a right-angled Artin group.
The main result of this paper is a complete classification of the outer automorphism groups of two-generator, one-relator groups with torsion. To this classification we apply recent algorithmic results of Dahmani--Guirardel, which yields an algorithm to compute the isomorphism class of the outer automorphism group of a given two-generator, one-relator group with torsion.
On Conjugacy of MASAs and the Outer Automorphism Group of the Cuntz Algebra
研究了Cuntz代数的外自同构群的结构以及与之密切相关的MASAa在O_n中的共轭性问题。特别地，我们展示了一个不可数的MASA族，通过Bogolubov自同构共轭到标准MASA D_n，它们不是D_n的内共轭。
Let $Γ$ be a crystallographic group of dimension $n,$ i.e. a discrete, cocompact subgroup of $\operatorname{Isom}(\mathbb{R}^n)$ = $O(n)\ltimes\mathbb{R}^n.$ For any $n\geq 2,$ we construct a crystallographic group with a trivial center and a trivial outer automorphism group.
Lifting finite groups of outer automorphisms of free groups, surface groups and their abelianizations
本文讨论了有限子群，特别是有限循环子群，关于自由群的自同构群与外自同构群、曲面群及其阿贝尔化之间的典范投影的提升可能性.
The so-called Tits class, associated to an adjoint absolutely almost simple algebraic group, provides a cohomological obstruction for this group to admit an outer automorphism. If the group has inner type, this obstruction is the only one. In this paper, we prove this is not the case for classical groups of outer type, except for groups of type $^2\mathsf{A}_n$ with $n$ even, or $n=5$. More precisely, we prove a descent theorem for exponent $2$ and degree $6$ algebras with unitary involution, which shows that their automorphism groups have outer automorphisms. In all other relevant classical t…
Outer Automorphism Groups of Ordered Permutation Groups
一个无限线性序集（S，<=）称为双齐次的，如果它的自同构群A（S）作用在它上面2-传递.我们证明了对于某个双齐次链（S，<=），任何群G都是自同构群A（S）的外自同构群G cong Out（A（S））.
On groups with a class-preserving outer automorphism
在1911年，伯恩赛德问是否存在团体，有一个外部自同构保持共轭类。两年后，他回答了自己的问题，构建了一个家庭的这种群体。利用MAGMA中的小群库，我们确定了所有阶数n < 512的具有这样一个自同构的群。我们的研究导致发现了四个新的无限族这样的群体，所有这些都是2群的coclass 4。
We prove that a "random" free group outer automorphism is an ageometric fully irreducible outer automorphism whose ideal Whitehead graph is a union of triangles. In particular, we show that its attracting (and repelling) tree is a nongeometric $\mathbb R$-tree all of whose branch points are trivalent
Erratum to: Homology stability for outer automorphism groups of free groups
本文纠正了前两位作者的论文《自由群的外自同构群的同调稳定性》中定理5的证明。
Compressed conjugacy and the word problem for outer automorphism groups of graph groups
它示出，对于图组（直角Artin组）的共轭问题，以及限制版本的同时共轭问题，可以在多项式时间内解决，即使输入的字表示在压缩形式。因此，它遵循的字问题的外部自同构群的图群可以解决在多项式时间。
An algorithm to decide if an outer automorphism is geometric
针对一般外自同构的几何性判定难题，本文提出一种构造性算法，可判定其是否由紧致曲面同胚实现并在存在时给出该同胚。
For a classical group $G$ of type $\mathsf D_n$ over a field $k$ of characteristic different from $2$, we show the existence of a finitely generated regular extension $R$ of $k$ such that $G$ admits outer automorphisms over $R$. Using this result and a construction of groups of type $\mathsf A$ from groups of type $\mathsf D$, we construct new examples of groups of type $^2\mathsf A_n$ with $n\equiv 3\bmod 4$ and the first examples of type $^2\mathsf A_n$ with $n\equiv 1\bmod 4$ $(n\geq 5)$ that are not $R$-trivial, hence not rational (nor stably rational).
The expansion factors of an outer automorphism and its inverse
秩为n的自由群F_n的全不可约外自同构φ有一个展开因子，它常常不同于φ的逆的展开因子。然而，我们证明了phi的展开因子的平方和与它的逆的平方和之间的比率是有界的，上面的常数仅取决于秩n。我们还证明了一个更一般的定理，它适用于F_n的任意外自同构及其逆，以及它们的展开因子的整个谱。
On the residual finiteness of outer automorphisms of relatively hyperbolic groups
证明了共轭可分相对双曲群的外自同构群的每个虚扭自由子群是剩余有限的。作为一个直接结果，我们得到极限群的外自同构群是剩余有限的。
Bestvina, Feighn and Handel proved that every subgroup of the outer automorphism group, $\textrm{Out}(F_n)$, of the free group of rank $n$ is either virtually finitely generated abelian or contains a nonabelian free group. In this note we consider the more general situation of the outer automorphism group $\textrm{Out}(G)$ of a finitely generated free-by-finite group $G$. We show that $\textrm{Out}(G)$ is translation discrete and that every subgroup of $\textrm{Out}(G)$ is either virtually finitely generated abelian or contains a nonabelian free group.
Constructing and Classifying Fully Irreducible Outer Automorphisms of Free Groups
本文的主要定理在Out（F_r）理论的背景下模拟了一个映射类群定理（H. Masur和J. Smillie），它精确地确定了哪些索引列表来自伪Anosov映射类。由于理想的怀特海图给出了一个更好的不变量在类似的设置一个完全不可约的外自同构，我们而是专注于确定哪些21个连接，无环，5顶点图是理想的怀特海图的年龄，完全不可约的外自同构的自由群的秩3。我们的主要定理通过证明恰好有18个图像这样产生来实现这一点。我们还给出了一种方法，用于确定某些并发症称为周期尼尔森路径，证明了存在方便分解的代表年龄，完全不可约外自同构具有连接，无环，（2 r-1）-顶点理想Whitehead图，并证明了一个标准，用于确定代表年龄，完全不可约外自同构。我们用来构造自由群的完全不可约外自同构的方法，以及我们的识别和分解技巧，可以用来扩展我们的主要定理，因为它们在任何秩中都是有效的。我们的证明方法主要依赖于Bestvina-Feigh-Handel训练轨道理论和吸引叠层理论。
We consider the action of an irreducible outer automorphism $ϕ$ on the closure of Culler--Vogtmann Outer space. This action has north-south dynamics and so, under iteration, points converge exponentially to $[T^ϕ_+]$. For each $N \geq 3$, we give a family of outer automorphisms $ϕ_k \in \textrm{Out}(\mathbb{F}_N)$ such that as, $k$ goes to infinity, the rate of convergence of $ϕ_k$ goes to infinity while the rate of convergence of $ϕ_k^{-1}$ goes to one. Even if we only require the rate of convergence of $ϕ_k$ to remain bounded away from one, no such family can be constructed when $N < 3$. Thi…
Every locally compact group is the outer automorphism group of a II$_1$ factor
本文证明每个局部紧第二可数群都可实现为某个 II$_1$ 因子的外自同构群，方法是通过证明其为中心化子群，从而推广了此前仅适用于全不连通群和紧群的结果。
For any finitely generated group $G$, two complexity functions $α_G$ and $β_G$ are defined to measure the maximal possible gap between the norm of an automorphism (respectively outer automorphism) of $G$ and the norm of its inverse. Restricting attention to free groups $F_r$, the exact asymptotic behaviour of $α_2$ and $β_2$ is computed. For rank $r\geqslant 3$, polynomial lower bounds are provided for $α_r$ and $β_r$, and the existence of a polynomial upper bound is proved for $β_r$.
Outer space and finiteness properties for symmetric automorphisms of RAAGs, and generalisations
本文为直角Artin群构造了对称外自同构群的可缩外空间骨架，并利用其证明置换给定有限共轭类集合的外自同构子群具有VF型有限性，同时确定了对称外自同构群的虚拟上同调维数。
We give a condition on the defining graph of a right-angled Artin group which implies its automorphism group is virtually indicable, that is, it has a finite-index subgroup that admits a homomorphism onto $\Z$. We use this as part of a criterion that determines precisely when the outer automorphism group of a right-angled Artin group defined on a graph with no separating intersection of links has property (T). As a consequence we also obtain a similar criterion for graphs in which each equivalence class under the domination relation of Servatius generates an abelian group.
We generalise the notion of a separating intersection of links (SIL) to give necessary and sufficient criteria on the defining graph $Γ$ of a right-angled Coxeter group $W_Γ$ so that its outer automorphism group is large: that is, it contains a finite index subgroup that admits the free group $F_2$ as a quotient. When $Out(W_Γ)$ is not large, we show it is virtually abelian. We also show that the same dichotomy holds for the outer automorphism groups of graph products of finite abelian groups. As a consequence, these groups have property (T) if and only if they are finite, or equivalently $Γ$ …
Automorphisms of graphs of groups with cyclic edge groups
本文刻画了边群为循环群、顶点群无挠且中心化子循环的单端群图基本群的外自同构群，证明其本质上由顶点群外自同构、广义鲍姆斯拉格-索利塔尔群外自同构及广义扭转共同构成。
A Presentation for the Group of Pure Symmetric Outer Automorphisms of a Given Splitting of a Free Product
本文通过证明自同构在“Outer Space”特定子复形上的作用是单连通的，并应用K. S. Brown定理，给出了自由积给定分裂的纯对称外自同构群的简洁展示。
Automorphism and outer automorphism groups of Right-angled Artin groups are not relatively hyperbolic
本文证明定义图至少含3个顶点的右角Artin群的自同构群，以及非实质上同构于单顶点或不连通定义图的外自同构群，均不是相对双曲的。
We investigate the combinatorial and geometric properties of automorphism groups of universal right-angled Coxeter groups, which are the automorphism groups of free products of copies of Z_2. It is currently an open question as to whether or not these automorphism groups have non-positive curvature. Analogous to Outer Space as a model for Out(F_n), we prove that the natural combinatorial and topological model for their outer automorphism groups can \emph{not} be given an equivariant CAT(0) metric. This is particularly interesting as there are very few non-trivial examples of proving that a mod…
Finitely presentable, non-Hopfian groups with Kazhdan's Property (T) and infinite outer automorphism group
我们给出了具有无限外自同构群的Kazhdan群的简单例子。这回答了一个问题的保林，独立回答奥利维尔和明智的完全不同的方法。作为（非半单）李群中的算术格，我们的例子也被给出。我们还利用Abels关于p-adic群的紧可表示性的结果来表示一个可表示的非Hopfian Kazhdan群。这回答了奥利维尔和怀斯的一个问题。
Right-angled Artin groups as finite-index subgroups of their outer automorphism groups
本文证明每个直角阿廷群都是另一个直角阿廷群的外自同构群的有限指数子群，并利用纯对称外自同构群给出显式构造。
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.…
The question of existence of outer automorphisms of a simple algebraic group $G$ arises naturally both when working with the Galois cohomology of $G$ and as an example of the algebro-geometric problem of determining which connected components of the automorphism group of $G$ have rational points. The existence question remains open only for four types of groups, and we settle one of the remaining cases, type $^3D_4$. The key to the proof is a Skolem-Noether theorem for cubic etale subalgebras of Albert algebras which is of independent interest. Necessary and sufficient conditions for a simply …
Automorphismes du groupe des automorphismes d'un groupe de Coxeter universel
本文利用自由积的Guirardel-Levitt外空间，确定了秩$n \geq 4$的泛Coxeter群的自同构群及外自同构群的外自同构群结构。
A note on separability in outer automorphism groups
本文提出了某些外自同构群子群可分性的判据，通过加强并推广Hagen和Sisto关于映射类群的结果，回答了他们的问题。
An ergodic action of the outer automorphism group of a free group
当n>2时，秩为n的自由群F_n的外自同构群在SU（2）-特征标簇Hom（F_n，SU（2））/SU（2）$上关于勒贝格测度类的作用是遍历的.
On the residual finiteness of outer automorphisms of relatively hyperbolic groups
证明了共轭可分双曲群的外自同构群的每个虚扭自由子群是剩余有限的。由此，我们能够证明：每一个可生成的富克斯群和每一个自由有限群的外自同构群都是剩余有限的.对于相对双曲群，我们也推广了主要结果。
Property $R_\infty$ for Artin and Coxeter groups whose outer automorphisms are length-preserving
该研究证明非球面Artin与Coxeter群的保长自同构具无限Reidemeister数，进而确立多类Artin、Coxeter群的$R_\infty$性质，还证明无限不可约非仿射强刚性Coxeter群的自同构群具acylindrically双曲性。
On minimal finite quotients of outer automorphism groups of free groups
证明了当n=3和4时，n阶自由群的外自同构群Out F_n的最小非交换有限因子群是线性群PSL_n（Z_2）（实际上，这对任意秩n > 2都成立）.本文还讨论了Aut F_n和Out F_n（n = 3和4）的低指数子群的计算结果。
Double cosets for $\SU(2)\times...\times\SU(2)$ and outer automorphisms of free groups
考虑$\SU(2)$的$n$拷贝的乘积关于对角子群的双陪集空间。我们得到了这个空间的参数化，Haar测度的径向部分，以及自由群$F_{n-1}$和$n-1$弦的辫群的外自同构群的作用的明确公式。
We show that there is no uniform upper bound on |Out(Aut(A))| when A ranges over all right-angled Artin groups. This is in contrast with the cases where A is free or free abelian: for all n, Dyer-Formanek and Bridson-Vogtmann showed that Out(Aut(F_n)) = 1, while Hua-Reiner showed |Out(Aut(Z^n)| = |Out(GL(n,Z))| < 5. We also prove the analogous theorem for Out(Out(A)). We establish our results by giving explicit examples; one useful tool is a new class of graphs called austere graphs.
Growth and order of automorphisms of free groups and free Burnside groups
证明了自由群的外自同构是指数增长的当且仅当它诱导自由Burnside群的无限级外自同构具有足够大的奇指数.
Relative outer automorphisms of free groups
设$A_1,...,A_k$是$F_n$的自由因子系统。相对自同构群$Aut(F_n;A_1,...,A_k)$是由限制到每个$A_i$的$F_n$的自同构是由$F_n$中的元素共轭给出的群。相对外自同构群被定义为$Out(F_n;A_1,...,A_k) = Aut(F_n;A_1,...,A_k)/Inn(F_n)$，其中$Inn(F_n)$是由所有内自同构给出的$Aut(F_n)$的正规子群。我们定义了一个可收缩的空间上的$Out(F_n;A_1,...,A_k)$有限稳定剂的作用，我们计算这个群体的虚拟上同调维数。
Automorphisms of two-dimensional RAAGs and partially symmetric automorphisms of free groups
计算了自由群的部分对称外自同构群的虚上同调维数。利用这一点，我们得到了二维直角Artin群的外自同构群的VCD的新的上界和下界。在定义图为树的直角群的情况下，边界一致。
We compute the homology groups $H_*(\operatorname{Out}(F_7);\mathbb Q)$ of the outer automorphism group of the free group of rank $7$. We produce in this manner the first rational homology classes of $\operatorname{Out}(F_n)$ that are neither constant ($*=0$) nor Morita classes ($*=2n-4$).
We construct the first known examples of infinite subgroups of the outer automorphism group of Out(A_Gamma), for certain right-angled Artin groups A_Gamma. This is achieved by introducing a new class of graphs, called focused graphs, whose properties allow us to exhibit (infinite) projective linear groups as subgroups of Out(Out(A_Gamma)). This demonstrates a marked departure from the known behavior of Out(Out(A_Gamma)) when A_Gamma is free or free abelian, as in these cases Out(Out(A_Gamma)) has order at most 4. We also disprove a previous conjecture of the second author, producing new exampl…
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.
We study the dilatation of outer automorphisms of right-angled Artin groups. Given a right-angled Artin group defined on a simplicial graph: $A(Γ) = \langle V | E \rangle$ and an automorphism $ϕ\in Out(A(Γ))$ there is a natural measure of how fast the length of a word of $A(Γ)$ grows after $n$ iterations of $ϕ$ as a function of $n$, which we call the dilatation of $w$ under $ϕ$. We define the dilatation of $ϕ$ as the supremum over dilatations of all $w \in A(Γ)$. Assuming that $ϕ$ is a pure and square map, we show that if the dilatation of $ϕ$ is positive, then either there exists a free abeli…
Realizing Non-Archimedean Polish Groups as Outer Automorphism Groups
通过小消去理论，构造可数离散群使得其外自同构群同构于任意给定的非阿基米德波兰群，并给出一个Borel映射。
This paper studies the rigidity properties of the abstract commensurator of the outer automorphism group of a universal Coxeter group of rank $n$, which is the free product $W_n$ of $n$ copies of $\mathbb{Z}/2\mathbb{Z}$. We prove that for $n\geq 5$ the natural map $\mathrm{Out}(W_n) \to \mathrm{Comm}(\mathrm{Out}(W_n))$ is an isomorphism and that every isomorphism between finite index subgroups of $\mathrm{Out}(W_n)$ is given by a conjugation by an element of $\mathrm{Out}(W_n)$.
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