An Automorphism of A Free Metabelian Group Without Fixed Points

自由双交换李代数的自同构，I

Automorphisms of free metabelian Lie algebras, I

本文研究了自由双交换李代数的自同构性质，证明了特定条件下自同构的 tame 性及部分自同构的几乎 tame 性，推翻了领域内长期存在的结论。

On the polynomial automorphisms of a group

We prove that if a group is nilpotent (resp. metabelian), then so is the subgroup of its automorphism group generated by all polynomial automorphisms.

Fixed points of endomorphisms of a free metabelian group

We consider IA-endomorphisms (i.e., Identical in Abelianization) of a free metabelian group of finite rank, and give a matrix characterization of their fixed points which is similar to (yet different from) the well-known characterization of eigenvectors of a linear operator in a vector space. We then use our matrix characterization to elaborate several properties of the fixed point groups of metabelian endomorphisms. In particular, we show that the rank of the fixed point group of an IA-endomorphism of the free metabelian group of rank $n \ge 2$ can be either equal to 0, 1, or greater than $(n…

Arithmetic monodromy actions on pro-metabelian fundamental groups of once-punctured elliptic curves

We prove structure theorems for the moduli stack of elliptic curves equipped with $G$-structures, where $G$ is a finite 2-generated metabelian group. In particular, we show that if $G$ has exponent $e$, then there is a subgroup $H\le GL_2(\mathbb{Z}/e)$ such that $G$-structures on elliptic curves $E$ are equivalent to "congruence structures of level $H$". Our methods are almost entirely group theoretic. Let $\widehat{M}$ denote the free profinite metabelian group of rank 2, then along the way we prove a decomposition of $Out(\widehat{M})$ as an internal semi-direct product of the subgroup of "…

Bounding the exponent of a finite group by the exponent of the automorphism group and a theorem of Schur

Assume $G$ is a finite $p$-group, and let $S$ be a Sylow $p$-subgroup of $\operatorname{Aut}(G)$ with $\exp(S)=q$. We prove that if $G$ is of class $c$, then $\exp(G)|p^{\ceil{\log_pc}}q^3$, and if $G$ is a metabelian $p$-group of class at most $2p-1$, then $\exp(G)|pq^3$.

Co-periodicity isomorphisms between forests of finite p-groups

Based on a general theory of descendant trees of finite p-groups and the virtual periodicity isomorphisms between the branches of a coclass subtree, the behavior of algebraic invariants of the tree vertices and their automorphism groups under these isomorphisms is described with simple transformation laws. For the tree of finite 3-groups with elementary bicyclic commutator quotient, the information content of each coclass subtree with metabelian mainline is shown to be finite. As a striking novelty in this paper, evidence is provided of co-periodicity isomorphisms between coclass forests which…

Finite non-metabelian Schur sigma-Galois groups of class field towers

For each odd prime p>=5, there exist finite p-groups G with derived quotient G/D(G)=C(p)xC(p) and nearly constant transfer kernel type k(G)=(1,2,...,2) having two fixed points. It is proved that, for p=7, this type k(G) with the simplest possible case of logarithmic abelian quotient invariants t(G)=(11111,111,21,21,21,21,21,21) of the eight maximal subgroups is realized by exactly 98 non-metabelian Schur sigma-groups S of order 7^11 with fixed derived length dl(S)=3 and metabelianizations S/D(D(S)) of order 7^7. For p=5, the type k(G) with t(G)=(2111,111,21,21,21,21) leads to infinitely many n…

The group of inertial automorphisms of an abelian group

We study the group $IAut(A)$ generated by the inertial automorphisms of an abelian group $A$, that is, automorphisms $γ$ with the property that each subgroup $H$ of $A$ has finite index in the subgroup generated by $H$ and $Hγ$. Clearly, $IAut(A)$ contains the group $FAut(A)$ of finitary automorphisms of $A$, which is known to be locally finite. In a previous paper, we showed that $IAut(A)$ is (locally finite)-by-abelian. In this paper, we show that $IAut(A)$ is also metabelian-by-(locally finite). In particular, $IAut(A)$ has a normal subgroup $Γ$ such that $IAut(A)/Γ$ is locally finite and $…

亚交换 Baumslag–Solitar 群中的二次方程

Quadratic equations in metabelian Baumslag-Solitar groups

本文研究亚交换 Baumslag–Solitar 群 $\mathbf{BS}(1,n)$ 上二次方程的丢番图问题，证明 $n\neq\pm1$ 时其为 NP 完全，并刻画可定向、不可定向等子类的复杂性。

自由代数自同构群的切李代数

Tangent Lie Algebras of Automorphism Groups of Free Algebras

本文为自由代数自同构群引入切李代数，证明其包含于常散度导子李代数，并利用该工具证明几乎所有已知野生自同构（除Nagata和Anick外）均为绝对野生，包括Bergman自同构及多数多项式幂零李代数簇中的自由代数。

Valuations, completions, and hyperbolic actions of metabelian groups

Actions on hyperbolic metric spaces are an important tool for studying groups, and so it is natural, but difficult, to attempt to classify all such actions of a fixed group. In this paper, we build strong connections between hyperbolic geometry and commutative algebra in order to classify the cobounded hyperbolic actions of numerous metabelian groups up to a coarse equivalence. In particular, we turn this classification problem into the problems of classifying ideals in the completions of certain rings and calculating invariant subspaces of matrices. We use this framework to classify the cobou…

可解群的特征标、Hilbert-Schmidt 稳定性与稠密周期测度

Characters of solvable groups, Hilbert-Schmidt stability and dense periodic measures

本文利用亚阿贝尔群与多循环群的特征标理论及紧阿贝尔群自同构动力学，证明多类可解群具有 Hilbert-Schmidt 稳定性。

Class-preserving automorphisms and the normalizer property for Blackburn groups

For a group $G$, let $U$ be the group of units of the integral group ring $\mathbb{Z}G$. The group $G$ is said to have the normalizer property if $\text{N}_U(G)=\text{Z}(U)G$. It is shown that Blackburn groups have the normalizer property. These are the groups which have non-normal finite subgroups, with the intersection of all of them being nontrivial. Groups $G$ for which class-preserving automorphisms are inner automorphisms, $\text{Out}_c(G)=1$, have the normalizer property. Recently, Herman and Li have shown that $\text{Out}_c(G)=1$ for a finite Blackburn group $G$. We show that $\text{ou…

Intersections of automorphism fixed subgroups in the free group of rank three

We show that in the free group of rank 3, given an arbitrary number of automorphisms, the intersection of their fixed subgroups is equal to the fixed subgroup of some other single automorphism.

A note on fixed point free automorphisms of groups related to finite fields

In this paper, we are not limited to considering semi-scalar automorphisms of G as Lindsay N. Childs in 2012 which is a specific case of Aut(G). Some new results in determining the fixed point free automorphisms of G are developed.

Several types of solvable groups as automorphism groups of compact Riemann surfaces

Let $X$ be a compact Riemann surface of genus $g\geq 2$. Let $Aut(X)$ be its group of automorphisms and $G\subseteq Aut(X)$ a subgroup. Sharp upper bounds for $|G|$ in terms of $g$ are known if $G$ belongs to certain classes of groups, e.g. solvable, supersolvable, nilpotent, metabelian, metacyclic, abelian, cyclic. We refine these results by finding similar bounds for groups of odd order that are of these types. We also add more types of solvable groups to that long list by establishing the optimal bounds for, among others, groups of order $p^m q^n$. Moreover, we show that Zomorrodian's bound…

An efficient algorithm for finding a basis of the fixed point subgroup of an automorphism of a free group

We prove that for any automorphism $α$ of a free group F of finite rank, one can efficiently compute a basis of the fixed point subgroup Fix(α).

自由积的不可约位移1自同构的不动点

Fixed points of irreducible, displacement one automorphisms of free products

该研究探讨群自由积分解对应的G-树形变空间上的外自同构作用，证明不可约位移1自同构在协体积1切片中恰有一个不动点，为相关群论研究提供结论支撑。

On the derivation algebra of the free Lie algebra and trace maps

We calculate the irreducible decomposition of the images of the Johnson homomorphisms of the automorphism group of a free group and a free metabelian group. We determine the abelianization of the derivation algebra of the Chen Lie algebra as a Lie algebra, and show that the abelianizaton is given by the degree one part and the Morita's trace maps. We also consider twisted cohomology groups of the automorphism group of a free nilpotent group. We show that the trace map for the exterior product defines a non-trivial twisted second cohomology class of it.

关于自动同构固定子群的有限性性质

On the finiteness properties of fixed subgroups of automorphisms

本文利用Sigma不变量研究自动同构固定子群的有限性性质，并在中心为直因子的情况下解答了Lei、Ma和Zhang提出的问题。

Profinite groups with an automorphism whose fixed points are right Engel

An element $g$ of a group $G$ is said to be right Engel if for every $x\in G$ there is a number $n=n(g,x)$ such that $[g,{}_{n}x]=1$. We prove that if a profinite group $G$ admits a coprime automorphism $φ$ of prime order such that every fixed point of $φ$ is a right Engel element, then $G$ is locally nilpotent.

Fixed subgroups of automorphisms of relatively hyperbolic groups

Let G be a finitely generated relatively hyperbolic group. We show that if no peripheral subgroup of G is hyperbolic relative to a collection of proper subgroups, then the fixed subgroup of every automorphism of G is relatively quasiconvex. It follows that the fixed subgroup is itself relatively hyperbolic with respect to a natural family of peripheral subgroups. If all peripheral subgroups of G are slender (respectively, slender and coherent), our result implies that the fixed subgroup of every automorphism of G is finitely generated (respectively, finitely presented). In particular, this hap…

关于有限群中被某些域自同构固定的不可约特征标的次数

On the degrees of irreducible characters fixed by some field automorphism in finite groups

本文证明了 Ito-Michler 定理的一个变体，研究了在阶为 $p$ 的 Galois 自同构 $σ$ 保持不变的不可约特征标次数不被素数 $p$ 整除时有限群的性质。

Free group automorphisms with many fixed points at infinity

A concrete family of automorphisms alpha_n of the free group F_n is exhibited, for any n > 2, and the following properties are proved: alpha_n is irreducible with irreducible powers, has trivial fixed subgroup, and has 2n-1 attractive as well as 2n repelling fixed points at bdry F_n. As a consequence of a recent result of V Guirardel there can not be more fixed points on bdry F_n, so that this family provides the answer to a question posed by G Levitt.

Maximal index automorphisms of free groups with no attracting fixed points on the boundary are Dehn twists

In this paper we define a quantity called the rank of an outer automorphism of a free group which is the same as the index introduced in [D. Gaboriau, A. Jaeger, G. Levitt and M. Lustig, `An index for counting fixed points for automorphisms of free groups', Duke Math. J. 93 (1998) 425-452] without the count of fixed points on the boundary. We proceed to analyze outer automorphisms of maximal rank and obtain results analogous to those in [D.J. Collins and E. Turner, `An automorphism of a free group of finite rank with maximal rank fixed point subgroup fixes a primitive element', J. Pure and App…

Frobenius groups of automorphisms with almost fixed point free kernel

Let $FH$ be a Frobenius group with kernel $F$ and complement $H$, acting coprimely on the finite solvable group $G$ by automorphisms. We prove that if $C_{G}(H)$ is of Fitting length $n$ then the index of the $n$-th Fitting subgroup $F_{n}(G)$ in $G$ is bounded in terms of $|C_{G}(F)|$ and $|F|.$ This generalizes a result of Khukhro and Makarenko \cite{k-m} which handles the case $n=1.$

广义Baumslag-Solitar群的固定子群

Fixed subgroups of generalised Baumslag-Solitar groups

本文研究广义Baumslag-Solitar群的自同构固定子群，刻画了在边稳定子严格包含于顶点稳定子条件下存在非有限生成固定子群的群，并证明有限阶自同构及树图情形下固定子群均有限生成。

On finite soluble groups with almost fixed-point-free automorphisms of non-coprime order

It is proved that if a finite $p$-soluble group $G$ admits an automorphism $φ$ of order $p^n$ having at most $m$ fixed points on every $φ$-invariant elementary abelian $p'$-section of $G$, then the $p$-length of $G$ is bounded above in terms of $p^n$ and $m$; if in addition the group $G$ is soluble, then the Fitting height of $G$ is bounded above in terms of $p^n$ and $m$. It is also proved that if a finite soluble group $G$ admits an automorphism $ψ$ of order $p^aq^b$ for some primes $p,q$, then the Fitting height of $G$ is bounded above in terms of $|ψ|$ and $|C_G(ψ)|$.

The Fitting height of finite groups with a fixed-point-free automorphism satisfying an identity

Motivated by classic theorems of Thompson and Berger on the Fitting height of finite groups with a fixed-point-free automorphism of coprime order, we conjecture that, for every non-zero polynomial $f(x) = a_0 + a_1 x + \cdots + a_d x^d \in \mathbb{Z}[x] $, there is an integer $k > 0$ with the following property. Let $G$ be a finite (solvable) group with a fixed-point-free automorphism $α$ satisfying $\gcd(|G|,k)= 1$ and $$\{ g^{a_0} \cdot α(g)^{a_1} \cdot α^2(g)^{a_2} \cdots α^d(g)^{a_d} | g \in G \} = \{1\}.$$ Then the Fitting height of $G$ is at most the number of irreducible factors of $f(x…

On supersolubility of finite groups admitting a Frobenius group of automorphisms with fixed-point-free kernel

Assume that a finite group $G$ admits a Frobenius group of automorphisms $FH$ with kernel $F$ and complement $H$ such that $C_{G}(F)=1$. In this paper, we investigate this situation and prove that if $C_G(H)$ is supersoluble and $C_{G'}(H)$ is nilpotent, then $G$ is supersoluble. Also, we show that $G$ is a Sylow tower group of a certain type if $C_{G}(H)$ is a Sylow tower group of the same type.

Mazurov关于自由Burnside群自同构不动点的问题

Mazurov's question on fixed points of automorphisms of free Burnside groups

本文否定回答了Mazurov关于自由Burnside群自同构不动点的问题，证明对任意奇数m≥3及足够大的素数周期q，循环置换生成元的m阶自同构无不动点。

Profinite groups and the fixed points of coprime automorphisms

The main result of the paper is the following theorem. Let $q$ be a prime and $A$ an elementary abelian group of order $q^3$. Suppose that $A$ acts coprimely on a profinite group $G$ and assume that $C_G(a)$ is locally nilpotent for each $a\in A^{\#}$. Then the group $G$ is locally nilpotent.

关于被某个域自同构固定的不可约特征标次数与p-可解群

On the degrees of irreducible characters fixed by some field automorphism, p-solvable groups

本文将实值不可约特征标奇次数蕴含正规Sylow 2-子群的结论推广到p-可解群，证明若p不整除被p阶域自同构固定的不可约特征标次数，则该群具有正规Sylow p-子群。

On finite groups with automorphisms whose fixed points are Engel

The main result of the paper is the following theorem. Let $q$ be a prime, $n$ a positive integer and $A$ an elementary abelian group of order $q^2$. Suppose that $A$ acts coprimely on a finite group $G$ and assume that for each $a\in A^{\#}$ every element of $C_{G}(a)$ is $n$-Engel in $G$. Then the group $G$ is $k$-Engel for some $\{n,q\}$-bounded number $k$.

Finite groups with coprime fixed-point-free automorphisms and applications

We study a class of finite groups $G$ which behave similarly to elementary abelian $p$-groups with $p$ prime, that is, there exists a subgroup $N$ such that all elements of $G\setminus N$ are conjugate or inverse-conjugate under $\Aut(G)$. In this paper, we show that such groups are solvable. As an application, we characterise a class of normal edge-transitive Cayley graphs of $G$, which are complete multipartite graphs.

环面纽结群自同构的不动点

Fixed Points of Automorphisms of Torus Knot Groups

本文完全分类了环面纽结群自同构的不动点子群，给出同构类型和显式生成元，并利用与原始作用相容的 Bass-Serre 树上的自同构群作用作为主要工具。

Fixed points of automorphisms of certain non-cyclic $p$-groups and the dihedral group

Let $G=\mathbf{Z}_{p} \oplus \mathbf{Z}_{p^2}$, where $p$ is a prime number. Suppose that $d$ is a divisor of the order of $G$. In this paper we find the number of automorphisms of $G$ fixing $d$ elements of $G$, and denote it by $θ(G,d)$. As a consequence, we prove a conjecture of Checco-Darling-Longfield-Wisdom. We also find the exact number of fixed-point-free automorphisms of the group $\mathbf{Z}_{p^{a}} \oplus \mathbf{Z}_{p^{b}}$, where $a$ and $b$ are positive integers with $a<b$. Finally, we compute $θ(D_{2q},d)$, where $D_{2q}$ is the dihedral group of order $2q$, $q$ is an odd prime …

On the influence of the fixed points of an automorphism to the structure of a group

Let $α$ be a coprime automorphism of a group $G$ of prime order and let $P$ be an $α$-invariant Sylow $p$-subgroup of $G$. Assume that $p\notin π(C_G(α))$. Firstly, we prove that $G$ is $p$-nilpotent if and only if $C_{N_G(P)}(α)$ centralizes $P$. In the case that $G$ is $Sz(2^r)$ and $PSL(2,2^r)$-free where $r=|α|$, we show that $G$ is $p$-closed if and only if $C_G(α)$ normalizes $P$. As a consequences of these two results, we obtain that $G\cong P\times H$ for a group $H$ if and only if $C_G(α)$ centralizes $P$. We also prove a generalization of the Frobenius $p$-nilpotency theorem for grou…

Inverse semigroups with idempotent-fixing automorphisms

A celebrated result of J. Thompson says that if a finite group $G$ has a fixed-point-free automorphism of prime order, then $G$ is nilpotent. The main purpose of this note is to extend this result to finite inverse semigroups. An earlier related result of B. H. Neumann says that a uniquely 2-divisible group with a fixed-point-free automorphism of order 2 is abelian. We similarly extend this result to uniquely 2-divisible inverse semigroups.

Profinite groups with an automorphism of prime order whose fixed points have finite Engel sinks

A right Engel sink of an element $g$ of a group $G$ is a set ${\mathscr R}(g)$ such that for every $x\in G$ all sufficiently long commutators $[...[[g,x],x],\dots ,x]$ belong to ${\mathscr R}(g)$. (Thus, $g$ is a right Engel element precisely when we can choose ${\mathscr R}(g)=\{ 1\}$.) We prove that if a profinite group $G$ admits a coprime automorphism $φ$ of prime order such that every fixed point of $φ$ has a finite right Engel sink, then $G$ has an open locally nilpotent subgroup. A left Engel sink of an element $g$ of a group $G$ is a set ${\mathscr E}(g)$ such that for every $x\in G$ a…

A note on non-inner automorphism conjecture

In this paper we prove that every $2$-generator finite $p$-group $G$ has a non-inner automorphism of order $p$ leaving $G^pγ_4(G)$ elementwise fixed ($p\ge 5$). Moreover, we prove a $2$-generator finite $3$-group satisfying $|Ω_1(Z_2(G))|=p^2$ has a non-inner automorphism of order $p$ leaving $G^pγ_3(G)$ elementwise fixed. As a consequence we prove the non-inner automorphism conjecture for every finite $p$-group of coclass $4$ ($p\ge 3$), and coclass $5$ ($p\ge 5$).

Engel-like conditions in fixed points of automorphisms of profinite groups

Let $q$ be a prime and $A$ an elementary abelian $q$-group acting as a coprime group of automorphisms on a profinite group $G$. We show that if $A$ is of order $q^2$ and some power of each element in $C_G(a)$ is Engel in $G$ for any $a\in A^{\#}$, then $G$ is locally virtually nilpotent. Assuming that $A$ is of order $q^3$ we prove that if some power of each element in $C_G(a)$ is Engel in $C_G(a)$ for any $a\in A^{\#}$, then $G$ is locally virtually nilpotent. Some analogues consequences of quantitative nature for finite groups are also obtained.

Finite 2-groups of Class 2 with Specific Automorphism Group

In this paper we classify all finite 2-groups of class 2 for which every automorphism of order 2 leaving the Frattini subgroup elementwise fixed is inner. We prove that every such group G is isomorphic to Q(n; r) = <a, b| a^{2n}= b^{2r}= 1; a^2^{n-r}= [a, b]> for some positive integers r; n such that 2 < 2r <= n; and every automorphism of Q(n; r) of order 2 leaving the Frattini subgroup elementwise fixed is inner.

On the exponent of a finite group admitting a fixed-point-free four-group of automorphisms

Let $A$ be a group isomorphic with either $S_4$, the symmetric group on four symbols, or $D_8$, the dihedral group of order 8. Let $V$ be a normal four-subgroup of $A$ and $α$ an involution in $A\setminus V$. Suppose that $A$ acts on a finite group $G$ in such a manner that $C_G(V)=1$ and $C_G(α)$ has exponent $e$. We show that if $A\cong S_4$ then the exponent of $G$ is $e$-bounded and if $A\cong D_8$ then the exponent of the derived group $G'$ is $e$-bounded. This work was motivated by recent results on the exponent of a finite group admitting an action by a Frobenius group of automorphisms.

Stabiliser of an Attractive Fixed Point of an IWIP Automorphism of a free product

For a group $G$ of finite Kurosh rank and for some arbiratily free product decomposition of $G$, $G = H_1 \ast H_2 \ast ... \ast H_r \ast F_q$, where $F_q$ is a finitely generated free group, we can associate some (relative) outer space $\mathcal{O}(G, \{H_1,..., H_r \})$. We define the relative boundary $\partial (G, \{ H_1, ..., H_r \}) = \partial(G, \mathcal{O}) $ corresponding to the free product decomposition, as the set of infinite reduced words (with respect to free product length). By denoting $Out(G, \{ H_1, ..., H_r \})$ the subgroup of $Out(G)$ which is consisted of the outer automo…

On profinite groups with automorphisms whose fixed points have countable Engel sinks

An Engel sink of an element $g$ of a group $G$ is a set ${\mathscr E}(g)$ such that for every $x\in G$ all sufficiently long commutators $[...[[x,g],g],\dots ,g]$ belong to ${\mathscr E}(g)$. (Thus, $g$ is an Engel element precisely when we can choose ${\mathscr E}(g)=\{ 1\}$.) It is proved that if a profinite group $G$ admits an elementary abelian group of automorphisms $A$ of coprime order $q^2$ for a prime $q$ such that for each $a\in A\setminus\{1\}$ every element of the centralizer $C_G(a)$ has a countable (or finite) Engel sink, then $G$ has a finite normal subgroup $N$ such that $G/N$ i…

Noninner Automorphisms of Order p for Finite p-Groups of Coclass 2

Every finite $p$-group of coclass 2 has a noninner automorphism of order $p$ leaving the center elementwise fixed.

Every group is the outer automorphism group of an HNN-extension of a fixed triangle group

Fix an equilateral triangle group $T_i=\langle a, b; a^i, b^i, (ab)^i\rangle$ with $i\geq6$ arbitrary. Our main result is: for every presentation $\mathcal{P}$ of every countable group $Q$ there exists an HNN-extension $T_{\mathcal{P}}$ of $T_i$ such that $\operatorname{Out}(T_{\mathcal{P}})\cong Q$. We construct the HNN-extensions explicitly, and examples are given. The class of groups constructed have nice categorical and residual properties. In order to prove our main result we give a method for recognising malnormal subgroups of small cancellation groups, and we introduce the concept of "m…

Fitting height of finite groups admitting a fixed-point-free automorphism satisfying an additional polynomial identity

Let $f(x)$ be a non-zero polynomial with integer coefficients. An automorphism $φ$ of a group $G$ is said to satisfy the elementary abelian identity $f(x)$ if the linear transformation induced by $φ$ on every characteristic elementary abelian section $S$ of $G$ is annihilated by $f(x)$. We prove that if a finite (soluble) group $G$ admits a fixed-point-free automorphism $φ$ satisfying an elementary abelian identity $f(x)$, where $f(x)$ is a primitive polynomial, then the Fitting height of $G$ is bounded in terms of $\operatorname{deg}(f(x))$. We also prove that if $f(x)$ is any non-zero polyno…

具有固定特征标度的群

Groups with a Fixed Character Degree

研究对于满足特定条件的正整数\(d\)、\(e\)，是否存在阶为\(d(d + e)\)的可解群\(G\)使\(d\)为其不可约特征标度，通过\(d(d + e)\)素因子给出算术刻画，受相关有限群研究推动。

On groups with automorphisms whose fixed points are Engel

We complete the study of finite and profinite groups admitting an action by an elementary abelian group under which the centralizers of automorphisms consist of Engel elements. In particular, we prove the following theorems. Let $q$ be a prime and $A$ an elementary abelian $q$-group of order at least $q^2$ acting coprimely on a profinite group $G$. Assume that all elements in $C_{G}(a)$ are Engel in $G$ for each $a\in A^{\#}$. Then $G$ is locally nilpotent (Theorem B2). Let $q$ be a prime, $n$ a positive integer and $A$ an elementary abelian group of order $q^3$ acting coprimely on a finite gr…

Finite groups and Lie rings with an automorphism of order $2^n$

Suppose that a finite group $G$ admits an automorphism $φ$ of order $2^n$ such that the fixed-point subgroup $C_G(φ^{2^{n-1}})$ of the involution $φ^{2^{n-1}}$ is nilpotent of class $c$. Let $m=|C_G(φ)|$ be the number of fixed points of $φ$. It is proved that $G$ has a characteristic soluble subgroup of derived length bounded in terms of $n,c$ whose index is bounded in terms of $m,n,c$. A similar result is also proved for Lie rings.

Parageometric outer automorphisms of free groups

We study those fully irreducible outer automorphisms phi of a finite rank free group F_r which are ``parageometric'', meaning that the attracting fixed point of phi in the boundary of outer space is a geometric R-tree with respect to the action of F_r, but phi itself is not a geometric outer automorphism in that it is not represented by a homemorphism of a surface. Our main result shows that the expansion factor of phi is strictly larger than the expansion factor of the inverse of phi. As corollaries (proved independently by Guirardel), the inverse of a parageometric outer automorphism is neit…

Finite $p$-groups of class 2 have noninner automorphisms of order $p$

We prove that for any prime number $p$, every finite non-abelian $p$-group $G$ of class 2 has a noninner automorphism of order $p$ leaving either the Frattini subgroup $Φ(G)$ or $Ω_1(Z(G))$ elementwise fixed.