Dimitri Leemans, Adrien Vandenschrick
For each prime power $q\geq 5$, we construct a rank four chiral polytope that has a group $PSL(3,q)$ as automorphism group and Schläfli type $\{q-1,\frac{2(q-1)}{(3,q-1)},q-1\}$. We also construct rank five polytopes for some values of $q$ and we show that there is no chiral polytope of rank at least six having a group $PSL(3,q)$ or $PSU(3,q)$ as automorphism group.
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2-(v,k,3) designs admitting an almost simple, flag-transitive automorphism group with socle PSL(2,q)
本文研究了允许特定自同构群的2-(v,k,3)设计的分类问题，重点在于其socle结构为PSL(2,q)的情况。
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…
We complete the description of automorphism groups of all nonsplit extensions of elementary abelian $2$-groups by $PSL_2(q)$, with $q$ odd, for an irreducible induced action. An application of this result to the theory of $π$-submaximal subgroups is given.
We classify the nonsplit extensions of elementary abelian $p$-groups by $PSL_2(q)$, with odd $p$ dividing $q-1$, for an irreducible induced action, calculate the relevant low-dimensional cohomology groups, and describe the automorphism groups of such extensions.
Polytopes are ubiquitous in different areas of mathematics. Gleason and Hubard established a factorisation theorem, stating that every abstract polytope has a unique factorisation into prime polytopes. We compute the automorphism group of a certain family of inductively constructed polytopes using the unique factorisation.
Automorphism Groups of the $PSL_2(q)$ Commuting Involution Graphs
本文研究了PSL_2(q)交换involution图的自同构群，通过分析共轭类X的性质，确定了该图的自同构群结构。
$2$-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。
All polytopes are coset geometries: characterizing automorphism groups of k-orbit abstract polytopes
本文证明任意抽象n-多面体均可作为陪集几何构造，通过生成元、关系和交条件刻画了k-轨道多面体自同构群，并证明对所有k≠2及n≥3存在具有布尔自同构群的k-轨道n-多面体。
We present a new algorithm to compute all the chiral polytopes that have a given group $G$ as full automorphism group. This algorithm uses a new set of generators that characterize the group, all of them except one being involutions. It permits to compute all chiral polytopes of groups that were previously unreachable by other known algorithms.
Four infinite families of chiral $3$-polytopes of type $\{4, 8\}$ with solvable automorphism groups
本研究构造了四类具有可解自同构群的{4,8}型手性3-多胞体无限族，部分回应了Schulte与Weiss、Pellicer提出的相关问题。
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 $(15, 8, 4)$.
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 $A_7$.
Symmetric polytopes whose automorphism groups are 2-groups
研究了具有2-群自同构群的多面体，证明了其自同构群的继承性及扩展性质。
String C-groups of 2-power order project onto a common string C-group
本文研究了2次幂阶的字符串C群的性质，发现了一个特殊的正则d-多面体C_d，其自同构群的阶为2^{2d-1}，并且是所有高阶正则d-多面体中具有最小旗数的唯一最小多面体，同时证明了该多面体的自同构群是所有有限2次幂阶字符串C群的商群，且所有具有2次幂阶自同构群的有限正则d-多面体都覆盖C_d。
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 $G/N \simeq A_5$.
In this paper we show that the rank of every chiral polytope having a Suzuki group as automorphism group is $3$. This gives a positive answer to a conjecture of Isabel Hubard and Dimitri Leemans.
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…
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.
Every Ree group $R(q)$, with $q\neq 3$ an odd power of 3, is the automorphism group of an abstract regular polytope, and any such polytope is necessarily a regular polyhedron (a map on a surface). However, an almost simple group $G$ with $R(q) < G \leq \mathsf{Aut}(R(q))$ is not a C-group and therefore not the automorphism group of an abstract regular polytope of any rank.
For each $d\geq 3$, $n \geq 10$, and $k_1, k_2, \ldots, k_{d-1}\geq 2$ with $k_1+k_2+\ldots+k_{d-1}\leq n-1$, we construct a regular $d$-polytope whose automorphism group is of order $2^n$ and whose Schläfli type is $\{2^{k_1},2^{k_2}, \ldots, 2^{k_{d-1}}\}$.
We study properties of the realizations of groups as the combinatorial automorphism group of a convex polytope. We show that for any non-abelian group $G$ with a central involution there is a centrally symmetric polytope with $G$ as its combinatorial automorphisms. We show that for each integer $n$, there are groups that cannot be realized as the combinatorial automorphisms of convex polytopes of dimension at most $n$. We also give an optimal lower bound for the dimension of the realization of a group as the group of isometries that preserves a convex polytope.
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。
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 detailed proof is given of a theorem describing the centraliser of a transitive permutation group, with applications to automorphism groups of objects in various categories of maps, hypermaps, dessins, polytopes and covering spaces, where the automorphism group of an object is the centraliser of its monodromy group. An alternative form of the theorem, valid for finite objects, is discussed, with counterexamples based on Baumslag--Solitar groups to show how it fails more generally. The automorphism groups of objects with primitive monodromy groups are described, as are those of non-connected …
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 $\overline T$.
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…
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.
We augment the list of finite universal locally toroidal regular polytopes of type {3,3,4,3,3} due to P.McMullen and E.Schulte, adding as well as removing entries. This disproves a related long-standing conjecture. Our new universal polytope is related to a well-known Y-shaped presentation for the sporadic simple group $Fi_{22}$, and admits $S_4\times O_8^+(2){:}S_3$ as the automorphism group. We also discuss further extensions of its quotients in the context of Y-shaped presentations. As well, we note that two known examples of finite universal polytopes of type {3,3,4,3,3} are related to Y-s…
Regular polytopes of rank $n/2$ for transitive groups of degree $n$
该研究针对次数为$n$的传递群对应的抽象正多胞体，探究秩为$n/2$（$n\neq14$）的情况，发现秩较最大值降低1时，正多胞体数量会大幅增加，补充了相关领域的研究结论。
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.…
Regular 3-polytopes of type $\{n,n\}$
本文对每个 $n\geq3$ 构造了含 $2^n n$ 个旗标、类型为 $\{n,n\}$ 的自对偶正则 3-多面体，并显式给出其半直积自同构群，解决了相关存在性问题。
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.
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。
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…
Two-orbit right-angled Polytopes and Hypertopes
本文针对腔室恰有两个轨道的拟阵与超拟阵，建立腔室系统商的通用构造框架，结合无acylindrically双曲群机制，构造出边标记为∞的2类双轨道超拟阵，所有构造对象均为无限的。
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。
Reflection groups and polytopes over finite fields, II
当对结晶学Coxeter群$Γ$的标准表示进行模奇素数$p$约化时，得到了$\mathbb{Z}_p$上某正交空间中的有限表示.如果$Γ$有一个字符串图，后一个群通常是一个有限正则多胞形的自同构群。在第一部分中，我们描述了这种结构的基础知识，并列举了与秩为3的群和球面型或欧几里得型群相关的多面体。在本文中，我们调查等家庭的多面体更一般的选择$Γ$，包括所有组的秩4。特别地，我们深入研究了它们的几何性质和相应的有限正交群的代数结构之间的相互作用。
Cyclohedra are a well-known infinite familiy of finite-dimensional polytopes that can be constructed from centrally symmetric triangulations of even-sided polygons. In this article we introduce an infinite-dimensional analogue and prove that the group of symmetries of our construction is a semidirect product of a degree 2 central extension of Thompson's infinite finitely presented simple group T with the cyclic group of order 2. These results are inspired by a similar recent analysis by the first author of the automorphism group of an infinite-dimensional associahedron.
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.
Commensurators of some non-uniform tree lattices and Moufang twin trees
Sh. Mozes证明了格${\rm PSL}_2 \bigl({\bf F}_p[t{}^{-1}] \bigr)$的扩张子在$p+1$的Bruhat-Tits树的全自同构群中是稠密的，后者的群比${\rm PSL}_2 \bigl({\bf F}_p((t)) \bigr)$大得多。由GA Margulis准则，这个密度是一个推广的算术结果。我们证明了在H. Bass和A.卢博茨基这个结果涵盖了许多由Moufang孪生树得到的格。
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-…
We study the subgroup structure of the semigroup of finitary tropical matrices under multiplication. We show that every maximal subgroup is isomorphic to the full linear automorphism group of a related tropical polytope, and that each of these groups is the direct product of the real numbers with a finite group. We also show that there is a natural and canonical embedding of each full rank maximal subgroup into the group of units of the semigroup of matrices over the tropical semiring with minus infinity. Our results have numerous corollaries, including the fact that every automorphism of a pr…
A Zero-One Law for Random Subgroups of some Totally Disconnected Groups
设A是由d个元素拓扑生成的局部紧群，且k>d.考虑Aut（F_k）通过预合成对标记的k-生成的稠密子群D_{k,A}：= {h：F_k --> A| h（F_k）在A中稠密。我们证明了两个简单的，全不连通的的局部紧群的家庭的遍历性。(i)A = PSL（2，K）其中K是非阿基米德局部域（特征不等于2），（ii）A = Aut^{0}（T）-（q+1）-正则树的保向自同构群，q > 1.相反，明斯基最近的一个结果表明，当A = PSL（2，R）或A = PSL（2，C）时，相同的作用不是遍历的。因此，如果K是一个局部域（具有不同于2的特征），Aut（F_k）对D_{k,PSL(2,K)}的作用是遍历的，对于任何k>2，当且仅当K是非阿基米德的。遍历性意味着每个“可测性质”对A的几乎每个k-生成稠密子群都成立或不成立。
We give examples of finite string $C$-groups (the automorphism groups of abstract regular polytopes) that have irreducible characters of real Schur index $2$. This answers a problem of Monson concerning these groups.
Fundamental polytope for the Weyl group acting on a maximal torus of a compact Lie group
本文研究了有限Weyl群在紧Lie群极大环面上的作用，将其转化为扩展仿射Weyl群在对偶环面子代数上的作用，利用极小权理论给出了基本多胞体的凸包与半空间交集描述及自包含证明，并推导了扩展Dynkin图自同构群的结构推论。
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…
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…
Scissors automorphism groups I: Homological stability and K-theory
本文证明在温和条件下剪刀自同构群的同调独立于对象，可由剪刀同余 K-理论谱表达，提供群论解释与计算方法，并在经典情形及后续中得出同调计算与结构结果。
String C-group representations of transitive Groups: a case study with degree $11$
本文以 11 次传递群为案例，用非计算机辅助方法证明具有秩 4 或 5 弦 C-群表示的偶传递群必同构于 PSL_2(11)，并提供了可推广到其他传递群研究的置换表示图技术与基础引理。
On medium-rank Lie primitive and maximal subgroups of exceptional groups of Lie type
本文研究特征p李型群H嵌入例外代数群G的李本原子群，排除PSL_2型，证明除少数例外（如E_8中的若干小群）外不存在，并推广到自同构稳定性和极大子群刻画，结合表示论与计算方法。
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.
Finite groups in which some particular non-nilpotent maximal invariant subgroups have indices a prime-power
本文研究互素自同构群作用下有限群的结构，证明在特定极大不变子群为同构单群直积、且不含某正规子群的非幂零极大不变子群均具素数幂指数且排除PSL_2(7)作为合成因子的条件下，该群必可解。
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。
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.
Permutation groups of prime power degree and $p$-complements
本文分类了具有素数幂次传递置换表示的几乎单群及其$p$-补，证明本原群有正则子群且同幂次忠实表示自同构等价，并构造了补不等价类任意多的例子。
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