Automorphisms of nonsplit extensions of 2-groups by $PSL_2(q)$

On finite groups with few automorphism orbits

Denote by $ω(G)$ the number of orbits of the action of $Aut(G)$ on the finite group $G$. We prove that if $G$ is a finite nonsolvable group in which $ω(G) \leqslant 5$, then $G$ is isomorphic to one of the groups $A_5,A_6,PSL(2,7)$ or $PSL(2,8)$. We also consider the case when $ω(G) = 6$ and show that if $G$ is a nonsolvable finite group with $ω(G) = 6$, then either $G \simeq PSL(3,4)$ or there exists a characteristic elementary abelian $2$-subgroup $N$ of $G$ such that $G/N \simeq A_5$.

Projective linear groups as automorphism groups of chiral polytopes

It is already known that the automorphism group of a chiral polyhedron is never isomorphic to $PSL(2,q)$ or $PGL(2,q)$ for any prime power $q$. In this paper, we show that $PSL(2,q)$ and $PGL(2,q)$ are never automorphism groups of chiral polytopes of rank at least $5$. Moreover, we show that $PGL(2,q)$ is the automorphism group of at least one chiral polytope of rank $4$ for every $q\geq5$. Finally, we determine for which values of $q$ the group $PSL(2,q)$ is the automorphism group of a chiral polytope of rank $4$, except when $q=p^d\equiv3\pmod{4}$ where $d>1$ is not a prime power, in which c…

具有基座为\(PSL(2,q)\)的旗传递自同构群的\(2 -\)设计

$2$-designs admitting a flag-transitive automorphism group with socle $PSL(2,q)$

研究了具有基座为\(PSL(2,q)\)（\(q = p^{f}\geq 4\)）的旗传递自同构群的\(2 -\)设计，分点本原和点非本原情况，后者获完全分类，前者除特定线性空间外还分类出\(48\)个零星例子。

$θ$-semisimple twisted conjugacy classes of type D in $\operatorname{PSL}_n(q)$

Let $p$ be an odd prime, $m\in {\mathbb N}$ and set $q=p^m$, $G=\operatorname{PSL}_n(q)$. Let $θ$ be a standard graph automorphism of $G$, $d$ be a diagonal automorphism and $\operatorname{Fr}_q$ be the Frobenius endomorphism of $\operatorname{PSL}_n(\overline{{\mathbb F}_q})$. We show that every $(d\circ θ)$-conjugacy class of a $(d\circ θ,p)$-regular element in $G$ is represented in some $\operatorname{Fr}_q$-stable maximal torus and that most of them are of type D. We write out the possible exceptions and show that, in particular, if $n\geq5$ is either odd or a multiple of $4$ and $q>7$, th…

The subgroup $PSL(2,R)$ is spherical in the group of diffeomorphisms of the circle

We show that the group $PSL(2,R)$ is a spherical subgroup in the group of $C^3$-diffeomorphisms of the circle. Also, the group of automorphisms of a Bruhat--Tits tree is a spherical subgroup in the group of hierarchomorphisms of the tree.

Classification of flag-transitive primitive symmetric $(v,k,λ)$ designs with $PSL(2,q)$ as socle

Let $\mathcal D$ be a nontrivial symmetric $(v,k,λ)$ design, and $G$ be a subgroup of the full automorphism group of $\mathcal D$. In this paper we prove that if $G$ acts flag-transitively, point-primitively on $\mathcal D$ and $Soc(G)= PSL(2,q)$, then D has parameters $(7, 3, 1)$, $(7, 4, 2)$, $(11, 5, 2)$, $(11, 6, 3)$ or $(15, 8, 4)$.

Chiral polyhedra and finite simple groups

We prove that every finite non-abelian simple group acts as the automorphism group of a chiral polyhedron, apart from the groups $PSL_2(q)$, $PSL_3(q)$, $PSU_3(q)$ and $A_7$.

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…

具有7阶自同构的三次四维簇

Cubic Fourfolds with an Order-$7$ Automorphism

本文研究含7阶自同构的光滑三次四维簇，确定其全部完全自同构群，并刻画两类模空间、边界奇异簇及单值群可公度类。

Commensurators of some non-uniform tree lattices and Moufang twin trees

Sh. Mozes showed that the commensurator of the lattice ${\rm PSL}_2 \bigl({\bf F}_p[t{}^{-1}] \bigr)$ is dense in the full automorphism group of the Bruhat-Tits tree of valency $p+1$, the latter group being much bigger than ${\rm PSL}_2 \bigl({\bf F}_p((t)) \bigr)$. By G.A. Margulis' criterion, this density is a generalized arithmeticity result. We show that the density of the commensurator holds for many tree-lattices among those called of Nagao type by H. Bass and A. Lubotzky. The result covers many lattices obtained via Moufang twin trees.

Embeddings of $\mathrm{PSL}_2(q)$ in exceptional groups of Lie type over a field of characteristic $\ne2,3$

Let $\boldsymbol{G}$ be an algebraic group of exceptional Lie type in characteristic $p$, $G=\boldsymbol{G}^σ$ its fixed-point subgroup under the action of a Steinberg endomorphism $σ$, and $\overline{G}$ an almost simple group with socle $G$. A maximal subgroup $M<\overline{G}$ is called non-generic if it is almost simple and its socle is not isomorphic to a group of Lie type in characteristic $p$. A finite subgroup $H$ of $\boldsymbol{G}$ is Lie primitive if it does not lie in any proper closed positive-dimensional subgroup of $\boldsymbol{G}$; it is Lie imprimitive if it lies in a positive-…

Automorphisms of $\overline{T}$

Let $\overline G$ be the wonderful compactification of a simple affine algebraic group $G$ defined over $\mathbb C$ such that its center is trivial and $G\not= {\rm PSL}(2,\mathbb{C})$. Take a maximal torus $T \subset G$, and denote by $\overline T$ its closure in $\overline G$. We prove that $T$ coincides with the connected component, containing the identity element, of the group of automorphisms of the variety $\overline T$.

Two groups 2^3.PSL_2(7) and 2^3:PSL_2(7) of order 1344

We analyze the group structures of two groups of order 1344 which are respectively non-split and split extensions of the elementary Abelian group of order 8 by its automorphism group PSL_2(7).They share the same character table. The group 2^3.PSL_2(7) is a finite subgroup of the Lie Group G_2 preserving the set of octonions \pm e_i , (i=1,2,...,7) representing a 7-dimensional octahedron.Its three maximal subgroups 2^3:7:3, 2^3.S_4 and 4.S_4:2 correspond to the finite subgroups of the Lie groups G_2, SO(4) and SU(3) respectively. The group 2^3:PSL_2(7) representing the split extension possesses…

On minimal finite quotients of outer automorphism groups of free groups

We prove that, for n=3 and 4, the minimal nonabelian finite factor group of the outer automorphism group Out F_n of a free group of rank n is the linear group PSL_n(Z_2) (conjecturally, this may remain true for arbitrary rank n > 2). We also discuss some computational results on low index subgroups of Aut F_n and Out F_n, for n = 3 and 4, using presentations of these groups.

On some rational extension properties for $GL_n(q)$ and even-degree characters fixed by order-2 Galois automorphisms

In this note, we prove that if every character of a finite group $G$ fixed by an order-2 Galois automorphism has odd degree, then $G$ has a normal Sylow $2$-subgroup. On the way, we study extensions of characters of $GL_n(q)$, $q$ odd, to the group extended by the transpose-inverse automorphism and prove that unipotent characters of $PSL_n(q)$ extend to rational characters of its automorphism group.

Regular maps with primitive automorphism groups

We classify the regular maps $\mathcal M$ which have automorphism groups $G$ acting faithfully and primitively on their vertices. As a permutation group $G$ must be of almost simple or affine type, with dihedral point stabilisers. We show that all such almost simple groups, namely all but a few groups ${\rm PSL}_2(q)$, ${\rm PGL}_2(q)$ and ${\rm Sz}(q)$, arise from regular maps, which are always non-orientable. In the affine case, the maps $\mathcal M$ occur in orientable and non-orientable Petrie dual pairs. We give the number of maps associated with each group, together with their genus and …

A Zero-One Law for Random Subgroups of some Totally Disconnected Groups

Let A be a locally compact group topologically generated by d elements and let k>d. Consider the action, by pre-composition, of Aut(F_k) on the set of marked, k-generated, dense subgroups D_{k,A} := {h:F_k --> A | h(F_k) is dense in A}. We prove the ergodicity of this action for two families of simple, totally disconnected locally compact groups. (i) A = PSL(2,K) where K is a non-Archimedean local field (of characteristic not equal to 2), (ii) A = Aut^{0}(T) - the group of orientation preserving automorphisms of a (q+1)-regular tree, for q > 1. In contrast, a recent result of Minsky's shows th…

Cubic graphical regular representations of some classical simple groups

A graphical regular representation (GRR) of a group $G$ is a Cayley graph of $G$ whose full automorphism group is equal to the right regular permutation representation of $G$. In this paper we study cubic GRRs of $\mathrm{PSL}_{n}(q)$ ($n=4, 6, 8$), $\mathrm{PSp}_{n}(q)$ ($n=6, 8$), $\mathrm{P}Ω_{n}^{+}(q)$ ($n=8, 10, 12$) and $\mathrm{P}Ω_{n}^{-}(q)$ ($n=8, 10, 12$), where $q = 2^f$ with $f \ge 1$. We prove that for each of these groups, with probability tending to $1$ as $q \rightarrow \infty$, any element $x$ of odd prime order dividing $2^{ef}-1$ but not $2^{i}-1$ for each $1 \le i < ef$ t…

On flag-transitive 2-(v,k,2) designs

This paper is devoted to the classification of flag-transitive 2-(v,k,2) designs. We show that apart from two known symmetric 2-(16,6,2) designs, every flag-transitive subgroup G of the automorphism group of a nontrivial 2-(v,k,2) design is primitive of affine or almost simple type. Moreover, we classify the 2-(v,k,2) designs admitting a flag transitive almost simple group G with socle PSL(n,q) for some n \geq 3. Alongside this analysis, we give a construction for a flag-transitive 2-(v,k-1,k-2) design from a given flag-transitive 2-(v,k,1) design which induces a 2-transitive action on a line.…

几乎单群作为λ=2的2-设计的旗传递自同构群

Almost simple groups as flag-transitive automorphism groups of 2-designs with λ = 2

本文研究λ=2的2-设计的旗传递几乎单自同构群，证明基群为有限李型例外单群时此类设计不存在，并完成相关设计的分类，明确其属于特定无限族或有限参数集的设计。

满足λ≥(r,λ)²的旗传递2-(v,k,λ)设计

Flag-transitive $2$-$(v,k,λ)$ designs with $λ\ge (r,λ)^2$

本文针对允许旗传递自同构群作用的满足λ≥(r,λ)²的2-设计，完成了n≥3时基座为PSLₙ(q)或PSUₙ(q)的几乎单群对应的满足λ≥(r,λ)²>1的该类设计的分类。

On spherical unitary representations of groups of spheromorphisms of Bruhat--Tits trees

Consider an infinite homogeneous tree $T_n$ of valence $n+1$, its group $Aut(T_n)$ of automorphisms, and the group $Hie(T_n)$ of its spheromorphisms (hierarchomorphisms), i.~e., the group of homeomorphisms of the boundary of $T_n$ that locally coincide with transformations defined by automorphisms. We show that the subgroup $Aut(T_n)$ is spherical in $Hie(T_n)$, i.~e., any irreducible unitary representation of $Hie(T_n)$ contains at most one $Aut(T_n)$-fixed vector. We present a combinatorial description of the space of double cosets of $Hie(T_n)$ with respect to $Aut(T_n)$ and construct a 'ne…

与二维射影特殊线性群相关的区传递 t-(k^2,k,λ) 设计

Block-transitive t-(k^2,k,λ) designs associated to two dimensional projective special linear groups

本文研究区传递自同构群作用下的 t-(k^2,k,λ) 设计，证明当正规子群为 PSL(2,q)（q≥4）时，仅当 q=8 时存在非平凡设计，且必为参数 λ∈{2,6,9,12,18,36} 的 2-(36,6,λ) 设计。

实现具有结构简化扩展的T-可解群的Gruenberg-Kegel图

Realizing Gruenberg-Kegel graphs of $T$-solvable groups with structurally simplified extensions of $T$

本文研究了T-可解群的素数图补集，证明其可由可解群和准简单T-可解群作用于元素阿贝尔群的直积实现，并探讨了PSL(2,13)-可解群的素数图补集分类。

Bowditch's Q-conditions and Minsky's primitive stability

For the action of the outer automorphism group of the rank two free group on the corresponding variety of PSL(2,C) characters, two domains of discontinuity have been known to exist that are strictly larger than the set of Schottky characters. One is introduced by Bowditch in 1998 (followed by Tan, Wong and Zhang in 2008) and the other by Minsky in 2013. We prove that these two domains are equal. We then show that they are contained in the set of characters having what we call the bounded intersection property.