Peter G. Doyle
I describe a 27-vertex graph that is vertex-transitive and edge-transitive but not 1-transitive. Thus while all vertices and edges of this graph are similar, there are no edge-reversing automorphisms.
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The main result of this paper is that, if $Γ$ is a connected 4-valent $G$-arc-transitive graph and $v$ is a vertex of $Γ$, then either $Γ$ is one of a well understood infinite family of graphs, or $|G_v|\leq 2^43^6$ or $2|G_v|\log_2(|G_v|/2)\leq |\VΓ|$ and that this last bound is tight. As a corollary, we get a similar result for $3$-valent vertex-transitive graphs.
We study vertex-quasiprimitive $2$-arc-transitive digraphs, reducing the problem of vertex-primitive $2$-arc-transitive digraphs to almost simple groups. This includes a complete classification of vertex-quasiprimitive $2$-arc-transitive digraphs where the action on vertices has O'Nan-Scott type SD or CD.
Vertex-transitive closures of graphs
研究图的顶点传递闭包及顶点传递数，运用代数和组合方法，确定使得图存在正则顶点传递闭包的最小整数，为相关研究提供方法与结论。
We introduce the concept of the primitivity of independent set in vertex-transitive graphs, and investigate the relationship between the primitivity and the structure of maximum independent sets in direct products of vertex-transitive graphs. As a consequence of our main results, we positively solve an open problem related to the structure of independent sets in powers of vertex-transitive graphs.
A core of a graph X is a vertex minimal subgraph to which X admits a homomorphism. Hahn and Tardif have shown that for vertex transitive graphs, the size of the core must divide the size of the graph. This motivates the following question: when can the vertex set of a vertex transitive graph be partitioned into sets each of which induce a copy of its core? We show that normal Cayley graphs and vertex transitive graphs with cores half their size always admit such partitions. We also show that the vertex sets of vertex transitive graphs with cores less than half their size do not, in general, ha…
We introduce uniformly vertex-transitive graphs as vertex-transitive graphs satisfying a stronger condition on their automorphism groups, motivated by a problem which arises from a Sinkhorn-type algorithm. We use the derangement graph $D(Γ)$ of a given graph $Γ$ to show that the uniform vertex-transitivity of $Γ$ is equivalent to the existence of cliques of sufficient size in $D(Γ)$. Using this method, we find examples of graphs that are vertex-transitive but not uniformly vertex-transitive, settling a previously open question. Furthermore, we develop sufficient criteria for uniform vertex-tra…
We solve the long-standing existence problem of vertex-primitive 2-arc-transitive digraphs by constructing an infinite family of such digraphs.
On the Induced Neighbourhood of Vertex-Transitive Graphs
本文提出新条件，排除大量图作为有限顶点传递图的诱导邻域，从而深化对顶点传递图结构限制的理解。
The Edge-Connectivity of Vertex-Transitive Hypergraphs
本文将 Mader 关于连通顶点传递图具有最大边连通度的经典定理推广到超图，证明连通线性一致顶点传递超图同样最大边连通，并说明线性或一致性条件不可或缺。
We prove that every regular graph of valency at least four whose automorphism group contains a nilpotent subgroup acting transitively on the vertex set admits a nowhere-zero 3-flow.
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.
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.
We describe two similar but independently-coded computations used to construct a complete catalogue of the transitive groups of degree less than $48$, thereby verifying, unifying and extending the catalogues previously available. From this list, we construct all the vertex-transitive graphs of order less than $48$. We then present a variety of summary data regarding the transitive groups and vertex-transitive graphs, focussing on properties that seem to occur most frequently in the study of groups acting on graphs. We illustrate how such catalogues can be used, first by finding a complete list…
For any finite, undirected, non-bipartite, vertex-transitive graph, we establish an explicit lower bound for the smallest eigenvalue of its normalised adjacency operator, which depends on the graph only through its degree and its isoperimetric constant.
We prove bounds on the chromatic number $χ$ of a vertex-transitive graph in terms of its clique number $ω$ and maximum degree $Δ$. We conjecture that every vertex-transitive graph satisfies $χ\le \max \left\{ω, \left\lceil\frac{5Δ+ 3}{6}\right\rceil\right\}$ and we prove results supporting this conjecture. Finally, for vertex-transitive graphs with $Δ\ge 13$ we prove the Borodin-Kostochka conjecture, i.e., $χ\le\max\{ω,Δ-1\}$.
It is well-known that concentrators are sparse graphs of high connectivity, which play a key role in the construction of switching networks; and any semi-vertex transitive graph is isomorphic to a bi-coset graph. In this paper, we prove that random bi-coset graphs are almost always concentrators, and construct some examples of semi-vertex transitive concentrators.
We investigate the structure of a digraph having a transitive automorphism group where every cutset of minimal cardinality consists of all successors or all predecessors of some vertex. We improve most of the existing results in this area.
A clique (resp., independent set) in a graph is strong if it intersects every maximal independent sets (resp., every maximal cliques). A graph is CIS if all of its maximal cliques are strong and localizable if it admits a partition of its vertex set into strong cliques. In this paper we prove that a clique $C$ in a vertex-transitive graph $Γ$ is strong if and only if $|C||I|=|V(Γ)|$ for every maximal independent set $I$ of $Γ$. Based on this result we prove that a vertex-transitive graph is CIS if and only if it admits a strong clique and a strong independent set. We classify all vertex-transi…
We generalise the standard constructions of a Cayley graph in terms of a group presentation by allowing some vertices to obey different relators than others. The resulting notion of presentation allows us to represent every vertex transitive graph. As an intermediate step, we prove that every countably infinite, connected, vertex transitive graph has a perfect matching. Incidentally, we construct an example of a 2-ended cubic vertex transitive graph which is not a Cayley graph, answering a question of Watkins from 1990.
Vertex-transitive graphs with small motion and transitive permutation groups with small minimal degree
本文研究图的自同构群的最小度（即图的运动量），通过分析本原和非本原置换群的小最小度，分类了运动量为4或素数的顶点传递图。
In 1979 Babai found a clever argument to prove that every connected vertex transitive graph on $n \ge 3$ vertices contains a cycle of length at least $\sqrt{3n}$. Here we modify his approach to show that such graphs must contain a cycle of length at least $(1 - o(1))n^{3/5}$.
It has long been known that there exist finite connected tetravalent arc-transitive graphs with arbitrarily large vertex-stabilisers. However, beside a well known family of exceptional graphs, related to the lexicographic product of a cycle with an edgeless graph on two vertices, only a few such infinite families of graphs are known. In this paper, we present two more families of tetravalent arc-transitive graphs with large vertex-stabilisers, each significant for its own reason.
For a non-complete graph $Γ$, a vertex triple $(u,v,w)$ with $v$ adjacent to both $u$ and $w$ is called a $2$-geodesic if $u\neq w$ and $u,w$ are not adjacent. Then $Γ$ is said to be $2$-geodesic transitive if its automorphism group is transitive on both arcs and 2-geodesics. In previous work the author showed that if a $2$-geodesic transitive graph $Γ$ is locally disconnected and its automorphism group $\Aut(Γ)$ has a non-trivial normal subgroup which is intransitive on the vertex set of $Γ$, then $Γ$ is a cover of a smaller 2-geodesic transitive graph. Thus the `basic' graphs to study are th…
A connected graph whose automorphism group acts transitively on the edges and vertices, but not on the set of ordered pairs of adjacent vertices of the graph is called half-arc-transitive. It is well known that the valence of a half-arc-transitive graph is even and at least four. Several infinite families of half-arc-transitive graphs of valence four are known, however, in all except four of the known specimens, the vertex-stabiliser in the automorphism group is abelian. The first example of a half-arc-transitive graph of valence four and with a non-abelian vertex-stabiliser was described in […
On cubic vertex-transitive graphs of given girth
针对立方点传递图的可区分代价研究空白，本文证明有两个边轨道且围长为5的该类图可区分代价为2，且不存在围长为6的无限3-弧传递立方图。
A graph of order $n$ is $p$-factor-critical, where $p$ is an integer of the same parity as $n$, if the removal of any set of $p$ vertices results in a graph with a perfect matching. 1-Factor-critical graphs and 2-factor-critical graphs are factor-critical graphs and bicritical graphs, respectively. It is well known that every connected vertex-transitive graph of odd order is factor-critical and every connected non-bipartite vertex-transitive graph of even order is bicritical. In this paper, we show that a simple connected vertex-transitive graph of odd order at least 5 is 3-factor-critical if …
The distinguishing number of a graph $G$ is the smallest $k$ such that $G$ admits a $k$-colouring for which the only colour-preserving automorphism of $G$ is the identity. We determine the distinguishing number of finite $4$-valent vertex-transitive graphs. We show that, apart from one infinite family and finitely many examples, they all have distinguishing number $2$.
It is shown that there are infinitely many connected vertex-transitive graphs that have no Hamilton decomposition, including infinitely many Cayley graphs of valency 6, and including Cayley graphs of arbitrarily large valency.
A step forward is made in a long standing Lovász's problem regarding hamiltonicity of vertex-transitive graphs by showing that every connected vertex-transitive graph of order a product of two primes, other than the Petersen graph, contains a Hamilton cycle. Essential tools used in the proof range from classical results on existence of Hamilton cycles, such as Chvátal's theorem and Jackson's theorem, to certain results on polynomial representations of quadratic residues at primitive roots in finite fields.
Highly regular vertex-transitive graphs are globally rigid
该研究证明了高度规则的顶点传递图在任何维度下都是全局刚性的，并展示了规则常数的最优性。
Vertex-transitive Neumaier graphs
本文研究顶点传递的诺伊迈尔图，给出其成为强正则图的充要条件，并识别度数不超过10的诺伊迈尔凯莱图。
Among all equivelar vertex-transitive maps on a given closed surface S, the automorphism groups of maps with Schläfli types {3, 7} and {7, 3} allow the highest possible order. We describe a procedure to transform all such maps into 1- or 2-orbit maps, whose symmetry type has been previously studied. In so doing we provide a procedure to determine all vertex-transitive maps with Schläfli type {3, 7} which are neither regular or chiral. We determine all such maps on surfaces with Euler characteristic -1 \geq \c{hi} \geq -40.
We characterise connected cubic graphs admitting a vertex- transitive group of automorphisms with an abelian normal subgroup that is not semiregular. We illustrate the utility of this result by using it to prove that the order of a semiregular subgroup of maximum order in a vertex-transitive group of automorphisms of a connected cubic graph grows with the order of the graph.
Cubic vertex-transitive graphs of girth six
本文完整分类了围长为6的有限简单三次点传递图，证明除Desargues图外，它们均可由环面六边形平铺、闭双曲曲面弧传递三角剖分的截断或6-正则图的弧传递二面体方案截断得到。
Half-arc-transitive graphs are a fascinating topic which connects graph theory, Riemann surfaces and group theory. Although fruitful results have been obtained over the last half a century, it is still challenging to construct half-arc-transitive graphs with prescribed vertex stabilizers. Until recently, there have been only six known connected tetravalent half-arc-transitive graphs with nonabelian vertex stabilizers, and the question whether there exists a connected tetravalent half-arc-transitive graph with nonabelian vertex stabilizer of order $2^s$ for every $s\geqslant3$ has been wide ope…
Determining the Locating Rainbow Connection Number of Vertex-Transitive Graphs
本研究围绕顶点传递图的定位彩虹连通数展开，明确该参数的定义内涵，推导出三个针对部分顶点传递图的相关定理，为该类图的彩虹连通性定位问题提供了理论结果。
A connected graph of order $n$ admitting a semiregular automorphism of order $n/k$ is called a $k$-multicirculant. Highly symmetric multicirculants of small valency have been extensively studied, and several classification results exist for cubic vertex- and arc-transitive multicirculants. In this paper we study the broader class of cubic vertex-transitive graphs of order $n$ admitting an automorphism of order $n/3$ or larger that may not be semiregular. In particular, we show that any such graph is either a $k$-multicirculant for some $k \leq 3$, or it belongs to an infinite family of graphs …
On Hamilton cycles in connected vertex-transitive graphs of order $2pq$
本文针对阶为$2pq$且满足特定自同构子群条件的连通点传递图，证明其均含哈密顿圈，为解决代数图论中相关公开问题提供了重要进展。
A finite graph is called a tricirculant if admits a cyclic group of automorphism which has precisely three orbits on the vertex-set of the graph, all of equal size. We classify all finite connected cubic vertex-transitive tricirculants. We show that except for some small exceptions of order less than 54, each of these graphs is either a prism of order 6k with k odd, a Möbius ladder, or it falls into one of two infinite families, each family containing one graph for every order of the form 6k with k odd.
Simple eigenvalues of cubic vertex-transitive graphs
本文研究了具有非平凡简单特征值的三次顶点传递图，发现其与弧传递性、正则地图和 Chebyshev 多项式存在显著联系。
Hamilton cycles in vertex-transitive graphs of order $6p$
本文研究阶为 $6p$（$p$ 为素数）的连通顶点传递图，证明除 Petersen 图的三角形替换图外，此类图均包含哈密顿圈，从而加强了已有哈密顿路径结果。
In this article we define a minor relation, which is stronger than the classical one, but too strong to become a well-quasi-order on the class of finite graphs. Nevertheless, with this terminology we are able to introduce a conjecture, which would imply the Lovasz conjecture and give an interesting insight on the symmetry of vertex-transitive graphs, if true. Though it could become an approach to solve the Lovasz conjecture. These ideas were first introduced by the author in his Diploma Thesis.
In this paper, we prove that the maximal order of a semiregular element in the automorphism group of a cubic vertex-transitive graph X does not tend to infinity as the number of vertices of X tends to infinity. This gives a solution (in the negative) to a conjecture of Peter Cameron, John Sheehan and the author. However, with an application of the positive solution of the restricted Burnside problem, we show that this conjecture holds true when X is either a Cayley graph or an arc-transitive graph.
Hamilton Cycles In Vertex-Transitive Graphs of Order 10p
本文证明了每个10p阶（p为素数）连通点传递图都包含哈密顿圈，唯一例外是Petersen图的截断，从而推进了该类图的哈密顿性问题研究。
Cubic vertex-transitive graphs of girth seven
本文通过图的签名对围长为7的三次点传递图进行完整分类，并证明若每条边包含在相同数量的7-圈中，则该图也是弧传递的。
In this paper, we study arc-transitive Cayley graphs on non-abelian simple groups with soluble stabilizers and valency seven. Let $\Ga$ be such a Cayley graph on a non-abelian simple group $T$. It is proved that either $\Ga$ is a normal Cayley graph or $\Ga$ is $S$-arc-transitive, with $(S,T)=(\A_n,\A_{n-1})$ and $n=7,21,63$ or $84$; and, for each of these four values of $n$, there really exists arc-transitive $7$-valent non-normal Cayley graphs on $\A_{n-1}$ and specific examples are constructed.
In interconnection networks, matching preclusion is a measure of robustness when there is a link failure. Let $G$ be a graph of even order. The matching preclusion number $mp(G)$ is defined as the minimum number of edges whose deletion results in a subgraph without perfect matchings. Many interconnection networks are super matched, that is, their optimal matching preclusion sets are precisely those induced by a single vertex. In this paper, we obtain general results of vertex-transitive graphs including many known networks. A $k$-regular connected vertex-transitive graph has matching preclusio…
In 2018 the first, Rukavina and the third author constructed with the aid of a computer the first example of a strongly regular graph $Γ$ with parameters (216, 40, 4, 8) and proved that it is the unique PSU(4,2)-invariant vertex-transitive graph on 216 vertices. In this paper, using the geometry of the Hermitian surface of PG(3, 4), we provide a computer-free proof of the existence of the graph $Γ$. The maximal cliques of $Γ$ are also determined.
Quasiprimitive and bi-quasiprimitive highly-arc-transitive digraphs and finite simple groups
本文研究了准素性和双准素性高度弧传递有向图与有限单群的关系，提出新的构造方法并揭示了相关开放问题。
Long Directed Cycles in Vertex-Transitive Digraphs
在图论与群论交叉领域，寻找顶点传递图和有向图中的哈密顿圈是经典问题。本文构造无限多个最长有向圈遗漏至少\(n/12\)个顶点的连通顶点传递有向图，证实相关猜想，还将有向圈长度下限从\(\Omega(n^{1/3})\)提高到\(\Omega(\sqrt n)\) 。
Gu et al. in [Inform. Process. Lett. 134 (2018) 52--56] conjectured that the data center network $D_{k,n}$ is vertex-transitive for all $k\geq0$ and $n\geq2$. In this paper, we show that $D_{k,n}$ is vertex-transitive for $k\leq1$ and $n\geq2$, and it is not vertex-transitive for all $k\geq2$ and $n\geq2$.
In the mid-1990s, two groups of authors independently obtained classifications of vertex-transitive graphs whose order is a product of two distinct primes. In the intervening years it has become clear that there is additional information concerning these graphs that would be useful, as well as making explicit the extensions of these results to digraphs. Additionally, there are several small errors in some of the papers that were involved in this classification. The purpose of this paper is to fill in the missing information as well as correct all known errors.
Linear circumference in vertex-transitive graphs
本文证明连通顶点传递图存在长度线性于顶点数的环，并给出大度数情形下的改进界，为Lovász哈密顿性猜想提供首个线性进展。
A graph of order $n$ is $p$-factor-critical, where $p$ is an integer of the same parity as $n$, if the removal of any set of $p$ vertices results in a graph with a perfect matching. 1-factor-critical graphs and 2-factor-critical graphs are well-known factor-critical graphs and bicritical graphs, respectively. It is known that if a connected vertex-transitive graph has odd order, then it is factor-critical, otherwise it is elementary bipartite or bicritical. In this paper, we show that a connected vertex-transitive non-bipartite graph of even order at least 6 is 4-factor-critical if and only if…
A graph $Γ$ is said to be symmetric if its automorphism group $\rm Aut(Γ)$ acts transitively on the arc set of $Γ$. In this paper, we show that if $Γ$ is a finite connected heptavalent symmetric graph with solvable stabilizer admitting a vertex-transitive non-abelian simple group $G$ of automorphisms, then either $G$ is normal in $\rm Aut(Γ)$, or $\rm Aut(Γ)$ contains a non-abelian simple normal subgroup $T$ such that $G\leq T$ and $(G,T)$ is explicitly given as one of $11$ possible exception pairs of non-abelian simple groups. Furthermore, if $G$ is regular on the vertex set of $Γ$ then the e…
A Cayley graph $\Ga=\Cay(G,S)$ is said to be normal if $G$ is normal in $\Aut\Ga$. The concept of normal Cayley graphs was first proposed by M.Y.Xu in [Discrete Math. 182, 309-319, 1998] and it plays an important role in determining the full automorphism groups of Cayley graphs. In this paper, we investigate the normality problem of the connected arc-transitive pentavalent Cayley graphs with soluble vertex stabilizer on finite nonabelian simple groups. We prove that all such graphs $\Ga$ are either normal or $G=\A_{39}$ or $\A_{79}$. Further, a connected arc-transitive pentavalent Cayley graph…
In \cite{Chan95}, the authors classified the 2-extendable abelian Cayley graphs and posed the problem of characterizing all 2-extendable Cayley graphs. We first show that a connected bipartite Cayley (vertex-transitive) graph is 2-extendable if and only if it is not a cycle. It is known that a non-bipartite Cayley (vertex-transitive) graph is 2-extendable when it is of minimum degree at least five \cite{sun}. We next classify all 2-extendable cubic non-bipartite Cayley graphs and obtain that: a cubic non-bipartite Cayley graph with girth $g$ is 2-extendable if and only if $g\geq 4$ and it does…
A pair of graphs $(Γ,Σ)$ is said to be stable if the full automorphism group of $Γ\timesΣ$ is isomorphic to the product of the full automorphism groups of $Γ$ and $Σ$ and unstable otherwise, where $Γ\timesΣ$ is the direct product of $Γ$ and $Σ$. In this paper, we reduce the study of the stability of any pair of regular graphs $(Γ,Σ)$ with coprime valencies and vertex-transitive $Σ$ to that of $(Γ,K_2)$. Since the latter is well studied in the literature, this enables us to determine the stability of any pair of regular graphs $(Γ,Σ)$ with coprime valencies in the case when $Σ$ is vertex-transi…
We study groups acting vertex-transitively on connected, trivalent graphs such that stabilizers of vertices are infinite. If the action is edge-transitive, we prove that the graph has to be a tree. We analyze the case where the action is not edge-transitive and fully classify the possible $2$-ended graphs. We draw connections to Willis' scale function and re-prove a result by Trofimov.
The direct product $G\times H$ of graphs $G$ and $H$ is defined by: \[V(G\times H)=V(G)\times V(H)\] and \[E(G\times H)=\left\{[(u_1,v_1),(u_2,v_2)]: (u_1,u_2)\in E(G) \mbox{\ and\ } (v_1,v_2)\in E(H)\right\}.\] In this paper, we will prove that the equality $$α(G\times H)=\max\{α(G)|H|, α(H)|G|\}$$ holds for all vertex-transitive graphs $G$ and $H$, which provides an affirmative answer to a problem posed by Tardif (Discrete Math. 185 (1998) 193-200). Furthermore, the structure of all maximum independent sets of $G\times H$ are determined.
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