Shenglin Zhou, Delu Tian
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 $(15, 8, 4)$.
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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。
This paper studies flag-transitive point-primitive non-symmetric $2$-($v,k,2$) designs. We prove that if $\mathcal{D}$ is a non-trivial non-symmetric $2$-$(v,k,2)$ design admitting a flag-transitive point-primitive automorphism group $G$ with $Soc(G)=A_{n}$ for $n\geq5$, then $\mathcal{D}$ is a $2$-$(6,3,2)$ or $2$-$(10,4,2)$ design.
The purpose of this paper is to classify all pairs $(\mathcal{D}, G)$, where $\mathcal{D}$ is a non-trivial $2$-$(v, k, 2)$ design, and $G\leq Aut(\mathcal{D})$ acts transitively on the set of blocks of $\mathcal{D}$ and primitively on the set of points of $\mathcal{D}$ with sporadic socle. We prove that there exists only one such pair $(\mathcal{D}, G)$ in which $\mathcal{D}$ is a $2$-$(176,8,2)$ design and $G=HS$, the Higman-Sims simple group.
Analysing 2-$(v,k,2)$ designs admitting a flag-transitive almost simple automorphism group with socle $PSL(2,q)$ by means of conics and hyperovals of $PG(2,q)$
本文利用 PG(2,q) 中的圆锥曲线与超卵形线，解决了两个未决情形，从而完成了基座为 PSL(2,q) 的旗传递 λ=2 的 2-设计分类。
$2$-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。
We present a partial classification of those finite linear spaces $\mathcal{S}$ on which an almost simple group $G$ with socle $PSL(3,q)$ acts line-transitively.
Sporadic simple groups as flag-transitive automorphism groups of symmetric designs
本文分类了以散在单群为基座且旗传递的对称设计，证明此类设计恰有七个，其中一个点非本原、其余点本原。
Classification of flag-transitive $2$-$(v, k, λ)$ designs with alternating group $A_n$($n \le 10$) as socle
本文分类了以交错群 $A_n$($n \le 10$) 为基座的旗传递点本原非平凡 $2$-$(v, k, λ)$ 设计，同构意义下共得到 87 个设计，完善了几乎单群作用下设计的分类理论。
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.…
This paper begins the classification of all edge-primitive 3-arc-transitive graphs by classifying all such graphs where the automorphism group is an almost simple group with socle an alternating or sporadic group, and all such graphs where the automorphism group is an almost simple classical group with a vertex-stabiliser acting faithfully on the set of neighbours.
In this article, we study $2$-designs with $\gcd(r,λ)=1$ admitting a flag-transitive almost simple automorphism group with socle a finite simple exceptional group of Lie type. We obtain four infinite families of such designs and provide some examples in each of these families.
Block-transitive t-(k^2,k,\lam) designs and simple exceptional groups of Lie type
针对非平凡t-(k²,k,λ)设计的区传递自同构群G，证明其基柱不可能为有限李型单例外群，为该类设计的自同构群结构研究提供关键结论。
Let $\mathcal{S}$ be a finite thick generalized quadrangle, and suppose that $G$ is an automorphism group of $\mathcal{S}$. If $G$ acts primitively on both the points and lines of $\mathcal{S}$, then it is known that $G$ must be almost simple. In this paper, we show that if the socle of $G$ is $\mathrm{PSL}(2,q)$ with $q\geq4$, then $q=9$ and $\mathcal{S}$ is the unique generalized quadrangle of order $2$.
This paper considers flag-transitive $4$-$(q+1,k,λ)$ designs with $λ\geq5$ and $q+1>k>4$. Let the automorphism group of a design $\cal D$ be a simple group $G=PSL(2,q)$. Depend on the fact that the setwise stabilizer $G_B$ must be one of twelve kinds of subgroups, up to isomorphism we get the following two results. (i) If $10\geq λ\geq 5$, then except $(G,G_x,G_B,k,λ)=(PSL(2,761),{E_{761}}\rtimes {C_{380}},S_4,24,7)$ or $(PSL(2,512),{E_{512}}\rtimes {C_{511}},{D_{18}},18,8)$ undecided, $\cal D$ is a $4$-$(24,8,5)$, $4$-$(9,8,5)$, $4$-$(8,6,6)$, $4$-$(10,9,6)$, $4$-$(9,6,10)$, $4$-$(9,7,10)$, $…
The intersection densities of transitive actions of $\operatorname{PSL}_{2}(q)$ with cyclic point stabilizers
本文利用顶点传递辅助图，完全确定了点稳定子共轭于 $\mathbb{Z}_p$ 的 $\operatorname{PSL}_{2}(q)$ 传递作用的交集密度，并针对共轭于 $\mathbb{Z}_r$ 的情况构造了最大相交集。
This paper determined all pairs $(\mathcal{D},G)$ where $\mathcal{D}$ is a non-symmetric 2-$(v,k,λ)$ design with $(r,λ)=1$ and $G$ is the almost simple flag-transitive automorphism group of $\mathcal{D}$ with an exceptional socle of Lie type. We prove that if $T\trianglelefteq G\leq Aut(T)$ where $T$ is an exceptional group of Lie type, then $T$ must be the Ree group or Suzuki group, and there are five classes of non-isomorphic designs $\mathcal{D}$.
Mathieu groups as flag-transitive automorphism groups of block designs
本文研究基座为 Mathieu 群的旗传递几乎单自同构群作用下的 2-设计，完整分类并显式构造，新得 101 个旗传递 2-设计。
Suppose that an automorphism group $G$ acts flag-transitively on a finite generalized hexagon or octagon $\cS$, and suppose that the action on both the point and line set is primitive. We show that $G$ is an almost simple group of Lie type, that is, the socle of $G$ is a simple Chevalley group.
A \textit{k}-arc in the projective space ${\rm PG}(n,q)$ is a set of $k$ projective points such that no subcollection of $n+1$ points is contained in a hyperplane. In this paper, we construct new $60$-arcs and $110$-arcs in ${\rm PG}(4,q)$ that do not arise from rational or elliptic curves. We introduce computational methods that, when given a set $\mathcal{P}$ of projective points in the projective space of dimension $n$ over an algebraic number field $\mathcal{Q}(ξ)$, determines a complete list of primes $p$ for which the reduction modulo $p$ of $\mathcal{P}$ to the projective space ${\rm PG…
We study $G$-vertex-primitive and $(G,s)$-arc-transitive digraphs for almost simple groups $G$ with socle $\mathrm{PSL}_n(q)$. It turns out that $s\leqslant2$ for such digraphs, which provides the first step in determining an upper bound on $s$ for all the vertex-primitive $s$-arc-transitive digraphs.
Let $G$ be a group with socle a simple group of Lie type defined over the finite field with $q$ elements where $q$ is a power of the prime $p$. Suppose that $G$ acts transitively upon the lines of a linear space $\mathcal{S}$. We show that if $p$ is {\it significant} then $G$ acts flag-transitively on $\mathcal{S}$ and all examples are known.
Alternating groups as flag-transitive automorphism groups of 2-designs with block size seven
该研究刻画了区组大小为7且自同构群为交错群型几乎单群的旗传递2-设计，证明其仅有两类可能情形。
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。
Let $p$ and $q$ be distinct odd primes. Let $Γ=(V(Γ), E(Γ))$ be a non-Cayley vertex-transitive graph of order $pq.$ Let $G\leq \Aut(Γ)$ acts primitively on the vertex set $V(Γ)$. In this paper, we show that $G$ is uniprimitive which is primitive but not 2-transitive and we obtain some information about $p, q$ and the minimality of the Socle $T=\soc(G).$
In this article, we study $2$-designs with $\gcd(r, λ)=1$ admitting a flag-transitive automorphism group. The automorphism groups of these designs are point-primitive of almost simple or affine type. We determine all pairs $(\mathcal{D}, G)$, where $\mathcal{D}$ is a $2$-design with $\gcd(r, λ)=1$ and $G$ is a flag-transitive almost simple automorphism group of $\mathcal{D}$ whose socle is $X=\mathrm{PSU}(n, q)$ with $(n, q)\neq (3, 2)$ and prove that such a design belongs to one of the two infinite families of Hermitian unitals and Witt-Bose-Shrikhande spaces, or it is isomorphic to a design …
On flag-transitive automorphism groups of $2$-designs with $λ$ prime
本文分类了具有素数λ且自同构群为旗传递点本原几乎单群的2-设计，证明了当基座为有限例外型单群时对应两类无限族设计，当基座为散在单群时对应三个唯一设计。
Block-transitive $3$-$(v,k,1)$ designs associated with alternating groups
本文证明：若块传递的 $3$-$(v,k,1)$ 设计具有几乎单自同构群且基座为交错群，则该设计必为唯一的 $3$-$(10,4,1)$ 设计，且群为 $\rm{PGL}(2,9)$、$\rm{M}_{10}$ 或 $\rm{Aut}(\rm{A}_6)$，并是旗传递的。
In 2017, Giudici, Li and the third author constructed the first known family of vertex-primitive $2$-arc-transitive digraphs of valency at least $2$. The smallest digraph in this family admits $\mathrm{PSL}_3(49)$ acting $2$-arc-transitively with vertex-stabilizer $\mathrm{A}_6$ and hence has $30758154560$ vertices. In this paper, we prove that this digraph is the vertex-primitive $2$-arc-transitive digraph of valency at least $2$ with fewest vertices.
Flag-transitive point-primitive quasi-symmetric $2$-designs and exceptional groups of Lie type
本文研究具有特定块相交数的非平凡拟对称2-设计的旗传递点本原自同构群，证明其基柱不可能是有限单例外李型群，推进了相关分类问题。
Flag-transitive point-primitive quasi-symmetric $2$-designs with block intersection numbers $0$ and $y\leq10$
研究旗传递、点本原且块交数为 $0$ 与 $2\leq y\leq10$ 的拟对称 $2$-设计，分类其自同构群类型，并排除交错群基柱、确定散在群情形。
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。
Maps, simple groups, and arc-transitive graphs
本文确定了有限几乎单群的因子分解条件，并分类了允许无核弧传递子群的映射嵌入图，除特殊情形外仅包括完全图、Johnson图及14个额外图，特殊情形下构造无穷多个嵌入。
Flag transitive geometries with trialities and no dualities coming from Suzuki groups
本文利用Suzuki群的室系统构造了首个旗传递且具有三元性而无二元性的秩三关联几何无限族，并构造了相应的正则超图族。
A graph is edge-transitive if its automorphism group acts transitively on the edge set. In this paper, we investigate the automorphism groups of edge-transitive graphs of odd order and twice prime valency. Let $Γ$ be a connected graph of odd order and twice prime valency, and let $G$ be a subgroup of the automorphism group of $\Ga$. In the case where $G$ acts transitively on the edges and quasiprimitively on the vertices of $\Ga$, we prove that either $G$ is almost simple or $G$ is a primitive group of affine type. If further $G$ is an almost simple primitive group then, with two exceptions, t…
Let $G$ be a permutation group on a set $Ω$ and recall that a base for $G$ is a subset of $Ω$ such that its pointwise stabiliser is trivial. In a recent paper, Burness and Giudici introduced the Saxl graph of $G$, denoted $Σ(G)$, with vertex set $Ω$ and two vertices adjacent if they form a base. If $G$ is transitive, then $Σ(G)$ is vertex-transitive and it is natural to consider its valency (which we refer to as the valency of $G$). In this paper we present a general method for computing the valency of any finite transitive group and we use it to calculate the exact valency of every primitive …
We classify non-complete prime valency graphs satisfying the property that their automorphism group is transitive on both the set of arcs and the set of $2$-geodesics. We prove that either $Γ$ is 2-arc transitive or the valency $p$ satisfies $p\equiv 1\pmod 4$, and for each such prime there is a unique graph with this property: it is a non-bipartite antipodal double cover of the complete graph $K_{p+1}$ with automorphism group $PSL(2,p)\times Z_2$ and diameter 3.
In this article, we study symmetric $(v, k, λ)$ designs admitting a flag-transitive and point-primitive automorphism group $G$ whose socle is a projective special unitary group of dimension at most five. We, in particular, determine all such possible parameters $(v, k, λ)$ and show that there exist eight non-isomorphic of such designs for which $λ\in\{3,6,12, 16, 18\}$ and $G$ is $PSU_{3}(3)$, $PSU_{3}(3):2$, $PSU_{4}(2)$ or $PSU_{4}(2):2$.
In this article, we investigate symmetric $(v,k,λ)$ designs $\mathcal{D}$ with $λ$ prime admitting flag-transitive and point-primitive automorphism groups $G$. We prove that if $G$ is an almost simple group with socle a finite simple group of Lie type, then $\mathcal{D}$ is either the point-hyperplane design of a projective space $\mathrm{PG}_{n-1}(q)$, or it is of parameters $(7,4,2)$, $(11,5,2)$, $(11,6,2)$ or $(45,12,3)$.
In this paper, we study intersecting sets in primitive and quasiprimitive permutation groups. Let $G \leqslant \mathrm{Sym}(Ω)$ be a transitive permutation group, and ${S}$ an intersecting set. Previous results show that if $G$ is either 2-transitive or a Frobenius group, then $|{S}|\leqslant|G_ω|$ (for some $ω\in Ω$). Furthermore, for some 2-transitive groups, $|{S}|=|G_ω|$ if and only if ${S}$ is a coset of a stabilizer. In this paper, we prove that these statements are far from the truth for general transitive groups. In particular, we show that in the case of primitive groups, there is eve…
We discuss recent progress on the problem of classifying point-primitive generalised polygons. In the case of generalised hexagons and generalised octagons, this has reduced the problem to primitive actions of almost simple groups of Lie type. To illustrate how the natural geometry of these groups may be used in this study, we show that if $\mathcal{S}$ is a finite thick generalised hexagon or octagon with $G \leqslant{\rm Aut}(\mathcal{S})$ acting point-primitively and the socle of $G$ isomorphic to ${\rm PSL}_n(q)$ where $n \geqslant 2$, then the stabiliser of a point acts irreducibly on the…
In this article, we study symmetric $(v, k, λ)$ designs admitting a flag-transitive and point-primitive automorphism group $G$ whose socle is $PSU_{4}(q)$. We prove that there exist eight non-isomorphic such designs for which $λ\in\{3,6,18\}$ and $G$ is either $PSU_{4}(2)$, or $PSU_{4}(2):2$.
The intersection density of cubic arc-transitive graphs with $2$-arc-regular full automorphism group equal to $\operatorname{PGL}_2(q)$
本文研究传递置换群的交密度问题，确定了具有2-弧-正则全自同构群$\operatorname{PGL}_2(q)$及弧-正则子群$\operatorname{PSL}_2(q)$的弧-传递图的自同构群的交密度。
Let $Γ$ be a finite X-symmetric graph with a nontrivial X-invariant partition $\mathcal {B}$ on $V(Γ)$ such that $Γ_{\mathcal {B}}$ is a connected (X,2)-arc-transitive graph and $Γ$ is not a multicover of $Γ_{\mathcal {B}}$. This article aims to give a characterization of $(Γ, X, \mathcal {B})$ for the case where $|Γ(C) \cap B| = 3$ for $B\in \mathcal {B}$ and $C \in Γ_{\mathcal {B}}(B)$. This investigation requires a study on (X,2)-arc-transitive graphs of valency 4 or 7. We give a characterization of tetravalent (X,2)-arc-transitive graphs at first; and as a byproduct, we prove that every te…
A graph $Γ$ is called $(G, s)$-arc-transitive if $G \le \mathrm{Aut}(Γ)$ is transitive on the set of vertices of $Γ$ and the set of $s$-arcs of $Γ$, where for an integer $s \ge 1$ an $s$-arc of $Γ$ is a sequence of $s+1$ vertices $(v_0,v_1,\ldots,v_s)$ of $Γ$ such that $v_{i-1}$ and $v_i$ are adjacent for $1 \le i \le s$ and $v_{i-1}\ne v_{i+1}$ for $1 \le i \le s-1$. $Γ$ is called 2-transitive if it is $(\mathrm{Aut}(Γ), 2)$-arc-transitive but not $(\mathrm{Aut}(Γ), 3)$-arc-transitive. A Cayley graph $Γ$ of a group $G$ is called normal if $G$ is normal in $\mathrm{Aut}(Γ)$ and non-normal othe…
A graph is said to be symmetric if its automorphism group is transitive on its arcs. Guo et al. (Electronic J. Combin. 18, \#P233, 2011) and Pan et al. (Electronic J. Combin. 20, \#P36, 2013) determined all pentavalent symmetric graphs of order $4pq$. In this paper, we shall generalize this result by determining all connected pentavalent symmetric graphs of order four times an odd square-free integer. It is shown in this paper that, for each of such graphs $\itΓ$, either the full automorphism group ${\sf Aut}\itΓ$ is isomorphic to ${\sf PSL}(2,p)$, ${\sf PGL}(2,p)$, ${\sf PSL}(2,p){\times}\mat…
A subset $\mathcal{F}$ of a finite transitive group $G\leq \operatorname{Sym}(Ω)$ is intersecting if for any $g,h\in \mathcal{F}$ there exists $ω\in Ω$ such that $ω^g = ω^h$. The \emph{intersection density} $ρ(G)$ of $G$ is the maximum of $\left\{ \frac{|\mathcal{F}|}{|G_ω|} \mid \mathcal{F}\subset G \mbox{ is intersecting} \right\}$, where $G_ω$ is the stabilizer of $ω$ in $G$. In this paper, it is proved that if $G$ is an imprimitive group of degree $pq$, where $p$ and $q$ are distinct odd primes, with at least two systems of imprimitivity then $ρ(G) = 1$. Moreover, if $G$ is primitive of de…
On the intersection density of the Kneser Graph $K(n,3)$
本文研究了 $n\geq 7$ 时 Kneser 图 $K(n,3)$ 的交集密度，确定了其自同构群包含 $\operatorname{PSL}_{2}(q)$ 时的交集密度，并简要探讨了 $K(n,2)$ 的相关情况。
The Erdős-Ko-Rado Theorem for non-quasiprimitive groups of degree $3p$
本文证明度为3p的非拟本原传递群的交密度必属于{1,3/2,3}，除非p为费马素数且诱导作用为包含PSL₂(q)的几乎单群。
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