Nick Gill
Suppose that a group $G$ acts transitively on the points of a non-Desarguesian plane, $\mathcal{P}$. We prove first that the Sylow 2-subgroups of $G$ are cyclic or generalized quaternion. We also prove that $\mathcal{P}$ must admit an odd order automorphism group which acts transitively on the set of points of $\mathcal{P}$.
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A long-standing conjecture is that any transitive finite projective plane is Desarguesian. We make a contribution towards a proof of this conjecture by showing that a group acting transitively on the the points of a non-Desarguesianprojective plane must not contain any components.
Suppose that a group $G$ acts transitively on the points of $\mathcal{P}$, a finite non-Desarguesian projective plane. We prove that if $G$ is insoluble then $G/O(G)$ is isomorphic to $SL_2(5)$ or $SL_2(5).2$.
We classify the pairwise transitive 2-designs, that is, 2-designs such that a group of automorphisms is transitive on the following five sets of ordered pairs: point-pairs, incident point-block pairs, non-incident point-block pairs, intersecting block-pairs and non-intersecting block-pairs. These 2-designs fall into two classes: the symmetric ones and the quasisymmetric ones. The symmetric examples include the symmetric designs from projective geometry, the 11-point biplane, the Higman-Sims design, and designs of points and quadratic forms on symplectic spaces. The quasisymmetric examples aris…
In this article we look into characterizing primitive groups in the following way. Given a primitive group we single out a subset of its generators such that these generators alone (the so-called primitive generators) imply the group is primitive. The remaining generators ensure transitivity or comply with specific features of the group. We show that, other than the symmetric and alternating groups, there are infinitely many primitive groups with one primitive generator each. These primitive groups are certain Mathieu groups, certain projective general and projective special linear groups, and…
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 a further contribution to the classification of line-transitive finite linear spaces. We prove that if S is a non-trivial finite linear space with the Fang-Li parameter gcd(k,r) is 9 or 10, the automorphism group G of S is line-transitive and point-imprimitive, then S is the Desarguesian projective plane PG(2,9).
Ostrom and Wagner (1959) proved that if the automorphism group $G$ of a finite projective plane $π$ acts $2$-transitively on the points of $π$, then $π$ is isomorphic to the Desarguesian projective plane and $G$ is isomorphic to $\mathrm{PΓL}(3,q)$ (for some prime-power $q$). In the more general case of a finite rank $2$ irreducible spherical building, also known as a \emph{generalized polygon}, the theorem of Fong and Seitz (1973) gave a classification of the \emph{Moufang} examples. A conjecture of Kantor, made in print in 1991, says that there are only two non-classical examples of flag-tra…
It is known that the Levi graph of any rank two coset geometry is an edge-transitive graph, and thus coset geometries can be used to construct many edge transitive graphs. In this paper, we consider the reverse direction. Starting from edge- transitive graphs, we construct all associated core-free, rank two coset geometries. In particular, we focus on 3-valent and 4-valent graphs, and are able to construct coset geometries arising from these graphs. We summarize many properties of these coset geometries in a sequence of tables; in the 4-valent case we restrict to graphs that have relatively sm…
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。
Rank three innately transitive permutation groups and related $2$-transitive groups
本文将秩至多为3的有限本原群和拟本原群分类推广到固有传递群，得到三个无限族和三个零散例子，并借助有限几乎单2-传递群中特殊对的确定完成分类。
A permutation group is called semiprimitive if each of its normal subgroups is either transitive or semiregular. Given nontrivial finite transitive permutation groups $L_1$ and $L_2$ with $L_1$ not semiprimitive, we construct an infinite family of rank two amalgams of permutation type $[L_1,L_2]$ and Borel subgroups of strictly increasing order. As an application, we show that there is no bound on the order of edge-stabilisers in locally $[L_1,L_2]$ graphs. We also consider the corresponding question for amalgams of rank $k\geq 3$. We completely resolve this by showing that the order of the Bo…
We prove that if $G$ is a group of finite Morley rank which acts definably and generically sharply $n$-transitively on a connected abelian group $V$ of Morley rank $n$ with no involutions, then there is an algebraically closed field $F$ of characteristic $\ne 2$ such that $V$ has a structure of a vector space of dimension $n$ over $F$ and $G$ acts on $V$ as the group $\operatorname{GL}_n(F)$ in its natural action on $F^n$. This is the final pre-publication version of the paper: A. Berkman and A. Borovik, Groups of finite Morley rank with a generically sharply multiply transitive action, J. Alg…
In this article, we investigate $2$-$(v,k,λ)$ designs with $\gcd(r,λ)=1$ admitting flag-transitive automorphism groups $G$. We prove that if $G$ is an almost simple group, then such a design belongs to one of the seven infinite families of $2$-designs or it is one of the eleven well-known examples. We describe all these examples of designs. We, in particular, prove that if $\mathcal{D}$ is a symmetric $(v,k,λ)$ design with $\gcd(k,λ)=1$ admitting a flag-transitive automorphism group $G$, then either $G\leq AΓL_{1}(q)$ for some odd prime power $q$, or $\mathcal{D}$ is a projective space or the …
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.…
We show that the geometry associated to certain non-split sharply 2-transitive groups does not contain a proper projective plane. For a sharply 2-transitive group of finite Morley rank we improve known rank inequalities for this geometry and conclude that a sharply 2-transitive group of Morley rank 6 must be of the form $K\rtimes K^*$ for some algebraically closed field $K$.
Finite semiprimitive permutation groups of rank $3$
本文给出了非固有传递的秩 $3$ 有限半本原置换群的完整分类，其例子包括某些几乎简单 $2$-传递群的 Schur 覆盖及三个例外小群。
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 $PSU_{4}(2):2$.
We investigate the configuration where a group of finite Morley rank acts definably and generically $m$-transitively on an elementary abelian $p$-group of Morley rank $n$, where $p$ is an odd prime, and $m\geqslant n$. We conclude that $m=n$, and the action is equivalent to the natural action of $\operatorname{GL}_n(F)$ on $F^n$ for some algebraically closed field $F$. This strengthens our earlier result in arXiv:1802.05222, and partially answers two problems posed in [9].
A simple undirected graph is weakly $G$-locally projective, for a group of automorphisms $G$, if for each vertex $x$, the stabiliser $G(x)$ induces on the set of vertices adjacent to $x$ a doubly transitive action with socle the projective group $L_{n_x}(q_x)$ for an integer $n_x$ and a prime power $q_x$. It is $G$-locally projective if in addition $G$ is vertex transitive. A theorem of Trofimov reduces the classification of the $G$-locally projective graphs to the case where the distance factors are as in one of the known examples. Although an analogue of Trofimov's result is not yet availabl…
Given a finite group $G$. The generating pair $(H,a)$ of $G$, that is, $H<G$ and $a\in G$ such that $\langle a,H\rangle=G$. In this paper, we introduce the definition of FF-subgroup to characterize the generating pairs of the symmetric groups, alternating groups and projective groups $PSL(2,q)$. This gives a partial answer to an open problem of J. André and J. Araújo and P. J. Cameron.
Given a linear order $Ω$ its automorphism group $\Aut(Ω)$ forms a lattice-ordered group via pointwise order. Assuming the continuum to be a regular cardinal, we show that \emph{pathological} and \emph{$ω$-transitive} (i.e. highly transitive) representations of free groups abound within \emph{large} permutation groups of linear orders. Consequently, under the Generalized Continuum Hypothesis it is then true that given any linear order $Ω$ for which $|Ω| = $ cof$(Ω) = \aleph_i$ ($i \in \N$) then any permutation group that is large in $\Aut(Ω)$ contains an $ω$-transitive representation of $G_{\al…
Families of Association Schemes on Triples from Two-Transitive Groups
本文研究双传递群作用产生的三元组结合方案，通过两点稳定子轨道确定其大小、第三价及交叉数，并证明射影群与零散群所得方案具有交换性。
Proper partial linear spaces affording imprimitive rank 3 automorphism groups
本研究针对真部分线性空间的对称分类问题，首次完成了半本原非本原秩3真部分线性空间的分类，涵盖本原传递、拟本原群情形，构造了多族无限实例及10个单独实例，多数为文献中新结果。
From the point of view of discrete geometry, the class of locally finite transitive graphs is a wide and important one. The subclass of Cayley graphs is of particular interest, as testifies the development of geometric group theory. Recall that Cayley graphs can be defined as non-empty locally finite connected graphs endowed with a transitive group action such that any non-identity element acts without fixed point. We define a class of transitive graphs which are transitive in an "absolutely non-Cayley way": we consider graphs endowed with a transitive group action such that any element of the…
The endomorphism rings of permutation modules of $\frac{3}{2}$-transitive permutation groups
本文针对 $\frac{3}{2}$-传递置换群的六类分类结果，通过逐类分析证明了其自然置换模的自同态环均为对称代数。
Affine groups as flag-transitive and point-primitive automorphism groups of symmetric designs
本文研究了容许旗传递且点本原仿射自同构群的对称设计，证明了当λ为素数时自同构群的结构，并给出了此类对称设计的完整分类。
In this note, we provide a short and self-contained proof that the braid group on n strands acts transitively on the set of reduced factorizations of a Coxeter element in a Coxeter group of finite rank n into products of reflections. We moreover use the same argument to also show that all factorizations of an element in a parabolic subgroup of W lie as well in this parabolic subgroup.
We show that the only transitive and generically $4$-transitive action of a group of finite Morley rank on a set of Morley rank $2$ is the natural action of $\operatorname{PGL}_3$ on the projective plane.
We classify C-groups of ranks $n-1$ and $n-2$ for the symmetric group $S_n$. We also show that all these C-groups correspond to hypertopes, that is, thin, residually connected flag-transitive geometries. Therefore we generalise some similar results obtained in the framework of string C-groups that are in one-to-one correspondence with abstract regular polytopes.
If $Γ$ is a string C-group which is isomorphic to a transitive subgroup of the symmetric group Sym(n) (other than Sym(n) and the alternating group Alt(n)), then the rank of $Γ$ is at most $n/2+1$, with finitely many exceptions (which are classified). It is conjectured that only the symmetric group has to be excluded.
We construct an infinite series of simply transitive irreducible lattices in PGL_2(F_q((t))) \times PGL_2(F_q((t))) by means of a quaternion algebra over F_q(t). The lattices depend on an odd prime power q = p^r and a parameter τ in F_q^* different from 1, and are the fundamental group of a square complex with just one vertex and universal covering T_{q+1} \times T_{q+1}, a product of trees with constant valency q + 1. Our lattices give rise via non-archimedian uniformization to smooth projective surfaces of general type over F_q((t)) with ample canonical class, Chern ratio (c_1)^2/c_2 = 2, tr…
Soft planes and groups
本文利用满足简单条件的群构造有限射影平面，所得平面包含许多已知平面并可能包含新平面，且精确刻画为具有固定旗并在特定旗集上传递的共线群的射影平面。
Let $N$ be a nilpotent group normal in a group $G$. Suppose that $G$ acts transitively upon the points of a finite non-Desarguesian projective plane $\mathcal{P}$. We prove that, if $\mathcal{P}$ has square order, then $N$ must act semi-regularly on $\mathcal{P}$. In addition we prove that if a finite non-Desarguesian projective plane $\mathcal{P}$ admits more than one nilpotent group which is regular on the points of $\mathcal{P}$ then $\mathcal{P}$ has non-square order and the automorphism group of $\mathcal{P}$ has odd order.
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。
Some remarks on the orbit dimension of transitive groups and on the metric dimension of Johnson graphs
研究传递群的轨道维数与Johnson图的度量维数，提出轨道维数的上界及等价条件，探讨对称群作用下轨道维数的计算方法。
Delandtsheer and Doyen bounded, in terms of the block size, the number of points of a point-imprimitive, block-transitive 2-design. To do this they introduced two integer parameters, m and n, now called Delandtsheer--Doyen parameters, linking the block size with the parameters of an associated imprimitivity system on points. We show that the Delandtsheer--Doyen parameters provide upper bounds on the permutation ranks of the groups induced on the imprimitivity system and on a class of the system. We explore extreme cases where these bounds are attained, give a new construction for a family of d…
Self-similarity of some soluble relatively free groups
研究可解相对自由群的传递自相似性，证明有限秩自由幂零群具有该性质，而秩至少为2的自由亚Abel群不具有。
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。
Groups Acting Generically Multiply Transitively on Solvable Groups
本文在有限 Morley 秩设定下，完成了群在可解群上泛多重传递作用的分类，证明此类作用等价于一般线性群在代数闭域向量空间上的自然作用，并推广了可定义本原群的分类。
The paper is an investigation of the structure of block-transitive automorphism groups of a 3-design with small block size. Let $G$ be a block-transitive automorphism group of a nontrivial $3$-$(v,k,λ)$ design $\mathcal{D}$ with $k\le 6$. We prove that if $G$ is point-primitive then $G$ is of affine or almost simple type. If $G$ is point-imprimitive then $\mathcal{D}$ is a $3$-$(16,6,λ)$ design with $λ\in\{4, 12, 16, 24, 28, 48, 56, 64, 84, 96, 112, 140\}$, and $rank(G)=3$.
The $S^2 \times R$ geometry can be derived by the direct product of the spherical plane $\bS^2$ and the real line $\bR$. J. Z. Farkas has classified and given the complete list of the space groups of $S^2 \times R$. The $S^2 \times R$ manifolds were classified by E. Molnár and J. Z. Farkas by similarity and diffeomorphism. In Szirmai we have studied the geodesic balls and their volumes in $S^2 \times R$ space, moreover we have introduced the notion of geodesic ball packing and its density and have determined the densest geodesic ball packing for generalized Coxeter space groups of $S^2 \times …
A classification is given of rank 3 group actions which are quasiprimitive but not primitive. There are two infinite families and a finite number of individual imprimitive examples. When combined with earlier work of Bannai, Kantor, Liebler, Liebeck and Saxl, this yields a classification of all quasiprimitive rank 3 permutation groups. Our classification is achieved by first classifying imprimitive almost simple permutation groups which induce a 2-transitive action on a block system and for which a block stabiliser acts 2-transitively on the block. We also determine those imprimitive rank 3 pe…
On finite permutation groups of rank three
本文研究有限非本原秩3置换群，针对块上诱导作用为仿射型的情形，依据块作用核将其分为四类，并完全分类一类半本那群、证明仿射情形下存在特殊p-群型正则正规子群。
Bounding the degree of generic sharp transitivity
本研究在L-群条件下证明了有限Morley秩泛型锐t-传递置换群的次数满足t≤r+2，推进了Borovik-Cherlin猜想，证明基于三个独立的群作用相关关键要素。
The Borovik-Cherlin conjecture holds in ACF
该研究证明了在代数闭域（ACF）中，连通群对不可约簇的特定置换作用同构于射影线性群的自然作用，确立了对应置换群的Borovik-Cherlin猜想。
Transitive fusion systems over a class of finite p-groups
本文针对有限 p-群上传递融合系统的分类猜想，证明当非交换有限 p-群 S 的 p-秩为 2 时，S 必为阶 p³ 的超特殊群。
Vertex-primitive s-arc-transitive digraphs admitting a Suzuki or Ree group
本文研究了容许Suzuki或Ree群的顶点本原s-弧传递有向图，证明了在此类群的情况下s≤1，并通过构造s=1的例子证明该界是紧的。
We show a simple geometric procedure for an extension of a loop realized as the image $Σ^{\ast}$ of a sharply transitive section in a subgroup $G^{\ast}$ of the projective linear group $PGL(n-1, \mathbb K)$ to a loop realized as the image of a sharply transitive section in a group $Δ=T' \rtimes C$ of affinities of the $n$-dimensional space ${\cal A}_n=\mathbb K^n$ over a commutative field $\mathbb K$. We desire that $T'$ is a large subgroup of affine translations and that $α(C)=G^{\ast}$ holds for the canonical homomorphism $α:GL(n,\mathbb K) \to PGL(n, \mathbb K)$. We demonstrate that our con…
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