Zejun Huang, Zhenhua Lyu
Let $P_{2,2}$ be the orientation of $C_4$ which consists of two 2-paths with the same initial and terminal vertices. In this paper, we determine the maximum size of $P_{2,2}$-free digraphs of order $n$ as well as the extremal digraphs attaining the maximum size when $n\ge 13$.
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In this paper, we characterize the extremal digraphs with the maximal or minimal $α$-spectral radius among some digraph classes such as rose digraphs, generalized theta digraphs and tri-ring digraphs with given size $m$. These digraph classes are denoted by $\mathcal{R}_{m}^k$, $\widetilde{\boldsymbolΘ}_k(m)$ and $\INF(m)$ respectively. The main results about spectral extremal digraph by Guo and Liu in \cite{MR2954483} and Li and Wang in \cite{MR3777498} are generalized to $α$-spectral graph theory. As a by-product of our main results, an open problem in \cite{MR3777498} is answered. Furthermo…
We comment on the paper "Extremal Cayley digraphs of finite Abelian groups" [Intercon. Networks 12 (2011), no. 1-2, 125--135]. In particular, we give some counterexamples to the results presented there, and provide a correct result for degree two.
In this paper, we determine the maximum size of digraphs on $n$ vertices in which there are no two distinct walks of length $3$ with the same initial vertex and the same terminal vertex. The digraphs attaining this maximum size are also characterized. Combining this with previous results, we obtain a full solution to a problem proposed by X. Zhan in 2007.
Let $n \ge 5$ and $k\ge 4$ be positive integers. We determine the maximum size of digraphs of order n that avoid distinct walks of length k with the same endpoints. We also characterize the extremal digraphs attaining this maximum number when $k \ge 5$.
Let $\overrightarrow{P_k}$ and $\overrightarrow{C_k}$ denote the directed path and the directed cycle of order $k$, respectively. In this paper, we determine the precise maximum size of $\overrightarrow{P_k}$-free digraphs of order $n$ as well as the extremal digraphs attaining the maximum size for large $n$. For all $n$, we also determine the precise maximum size of $\overrightarrow{C_k}$-free digraphs of order $n$ as well as the extremal digraphs attaining the maximum size. In addition, Huang and Lyu [\textit{Discrete Math. 343(5) 2020}] characterized the extremal digraphs avoiding an orient…
Let $n,k,t$ be positive integers. What is the maximum number of arcs in a digraph on $n$ vertices in which there are at most $t$ distinct walks of length $k$ with the same endpoints? In this paper, we prove that the maximum number is equal to $n(n-1)/2$ and the extremal digraph are the transitive tournaments when $k\ge n-1\ge \max\{2t+1,2\left\lceil \sqrt{2t+9/4}+1/2\right\rceil+3\}$. Based on this result, we may determine the maximum numbers and the extremal digraphs for $k\ge \max\{2t+1,2\left\lceil \sqrt{2t+9/4}+1/2\right\rceil+3\}$ and $n$ is sufficiently large, which generalises the exist…
Let $D=(V,A)$ be a digraphs without isolated vertices. A vertex-degree based invariant $I(D)$ related to a real function $φ$ of $D$ is defined as a summation over all arcs, $I(D) = \frac{1}{2}\sum_{uv\in A}{φ(d_u^+,d_v^-)}$, where $d_u^+$ (resp. $d_u^-$) denotes the out-degree (resp. in-degree) of a vertex $u$. In this paper, we give the extremal values and extremal digraphs of $I(D)$ over all digraphs with $n$ non-isolated vertices. Applying these results, we obtain the extremal values of some vertex-degree based topological indices of digraphs, such as the Randić index, the Zagreb index, the…
Extremal Laplacian energy of $\overrightarrow{C_{k+1}}$-free digraphs
本文研究了无向图 $\overrightarrow{C_{k+1}}$-free 有向图的最大拉普拉斯能量，并确定了极值图的结构。
Walks in a directed graph can be given a partially ordered structure that extends to possibly unconnected objects, called hikes. Studying the incidence algebra on this poset reveals unsuspected relations between walks and self-avoiding hikes. These relations are derived by considering truncated versions of the characteristic polynomial of the weighted adjacency matrix, resulting in a collection of matrices whose entries enumerate the self-avoiding hikes of length $\ell$ from one vertex to another.
Extremal digraphs on Meyniel-type condition for hamiltonian cycles in balanced bipartite digraphs
本文研究了平衡二分有向图中哈密顿圈的Meyniel型条件，刻画了满足度条件 $d(u)+d(v)\ge 3a$ 的极值有向图的两类结构，并证明了在条件放宽至 $3a-1$ 时图可迹，且下界紧。
Extremal Problems for the Family of $k$-Strongly Connected Digraphs
研究k-强连通有向图族的饱和数和极值数，推导出饱和数公式并给出极值数上界，提出关于极值数的猜想。
This paper presents a sharp upper bound for the spectral radius of simple digraphs with described number of arcs. Further, the extremal graphs which attain the maximum spectral radius among all simple digraphs with fixed arcs are investigated. In particular, we characterize all extremal simple digraphs with the maximum spectral radius among all simple digraphs with arcs number $e=2{k\choose 2}+t$ and $k>4t^4+4$.
An identifying code of a (di)graph $G$ is a dominating subset $C$ of the vertices of $G$ such that all distinct vertices of $G$ have distinct (in)neighbourhoods within $C$. In this paper, we classify all finite digraphs which only admit their whole vertex set in any identifying code. We also classify all such infinite oriented graphs. Furthermore, by relating this concept to a well known theorem of A. Bondy on set systems we classify the extremal cases for this theorem.
Let $G$ be a digraph with adjacency matrix $A(G)$ and outdegrees diagonal matrix $D(G)$. For any real $α\in[0,1]$, the $A_α$ matrix $A_α(G)$ of a digraph $G$ is defined as $A_α(G)=αD(G)+(1-α)A(G)$. The eigenvalue of $A_α(G)$ with the largest modulus is called the $A_α$ spectral radius of $G$. In this paper, we first give some upper bounds for the $A_α$ spectral radius of a digraph and we also characterize the extremal digraphs attaining these bounds. Moreover, we define the $A_α$ energy of a digraph $G$ as $E^{A_α}(G)=\sum\limits_{i=1}^n(λ^α_i(G))^2$, where $n$ is the number of vertices and $λ…
For a fixed simple digraph $F$ and a given simple digraph $D$, an $F$-free $k$-coloring of $D$ is a vertex-coloring in which no induced copy of $F$ in $D$ is monochromatic. We study the complexity of deciding for fixed $F$ and $k$ whether a given simple digraph admits an $F$-free $k$-coloring. Our main focus is on the restriction of the problem to planar input digraphs, where it is only interesting to study the cases $k \in \{2,3\}$. From known results it follows that for every fixed digraph $F$ whose underlying graph is not a forest, every planar digraph $D$ admits an $F$-free $2$-coloring, a…
Let $D$ be an directed graph on $p\geq 10$ vertices with minimum degree at least $p-1$ and minimum semi-degree at least $ p/2 -1$. We present a detailed proof of the following result [13]: The digraph $D$ is pancyclic, unless some extremal cases (which are characterized).
Extremal digraphs for open neighbourhood location-domination and identifying codes
本文研究开邻域定位支配集等于全部顶点的极值有向图，证明其一般结构性质并给出构造方法，同时为已有结果提供新证明并刻画极值有向树。
Extremal digraphs containing at most $t$ paths of length 2 with the same endpoints
本文研究了阶为 $n$ 的 $P_{t+1,2}$-free 有向图的最大规模，证明了对于足够大的 $n$，该极值在 $\lfloor(n-t)/{2} \rfloor$ 的奇偶性下分别等于 $g(n,t)$ 或在 $\{g(n,t)-1, g(n,t)\}$ 中取值。
Let $D$ be a strong digraph on $n=2m+1\geq 5$ vertices. In this paper we show that if $D$ contains a cycle of length $n-1$, then $D$ has also a cycle which contains all vertices with in-degree and out-degree at least $m$ (unless some extremal cases).
Recently, Grytczuk, Kordulewski, and Niewiadomski defined an extremal word over an alphabet $\mathbb{A}$ to be a word with the property that inserting any letter from $\mathbb{A}$ at any position in the word yields a given pattern. In this paper, we determine the number of extremal $XY_1XY_2X\dots XY_tX$-avoiding words on a $k$-letter alphabet. We also derive a lower bound on the shortest possible length of an extremal square-free word on a $k$-letter alphabet that grows exponentially in $k$.
Let $D$ be a digraph on $p\geq 5$ vertices with minimum degree at least $p-1$ and with minimum semi-degree at least $p/2-1$. For $D$ (unless some extremal cases) we present a detailed proof of the following results [12]: (i) $D$ contains cycles of length 3, 4 and $p-1$; (ii) if $p=2n$, then $D$ is hamiltonian.
In 1984, Plesník determined the minimum total distance for given order and diameter and characterized the extremal graphs and digraphs. We prove the analog for given order and radius, when the order is sufficiently large compared to the radius. This confirms asymptotically a conjecture of Chen et al. We also state an analog of the conjecture of Chen et al for digraphs and prove it for sufficiently large order.
Extremal problems in uniformly dense hypergraphs and digraphs
本文研究了3-图的均匀Turán密度，通过建立有向图极值问题与3-图均匀Turán密度之间的联系，给出了π_u(F)的可验证条件，并构造了满足特定密度的3-图实例。
Let $\mathcal{G}_{n,p,q}$ denote the set of strongly connected bipartite digraphs on $n$ vertices which contain a complete bipartite subdigraph $\overleftrightarrow{K_{p,q}}$, where $p, q, n$ are positive integers and $p+q \leq n$. In this paper, we study the Q-index (i.e. the signless Laplacian spectral radius) of $\mathcal{G}_{n,p,q}$, and determine the extremal digraph that has the minimum Q-index.
Bounds for the trace norm of $A_α$ matrix of digraphs
本文研究有向图 $A_α$ 矩阵的迹范数，通过求解基本有向图的奇异值并刻画秩为1的情形，给出了迹范数的下界与上界，并确定了相应的极值有向图和有向树。
For a digraph $D=(V(D), A(D))$, and a set $S\subseteq V(D)$ with $r\in S$ and $|S|\geq 2$, an $(S, r)$-tree is an out-tree $T$ rooted at $r$ with $S\subseteq V(T)$. Two $(S, r)$-trees $T_1$ and $T_2$ are said to be arc-disjoint if $A(T_1)\cap A(T_2)=\emptyset$. Two arc-disjoint $(S, r)$-trees $T_1$ and $T_2$ are said to be internally disjoint if $V(T_1)\cap V(T_2)=S$. Let $κ_{S,r}(D)$ and $λ_{S,r}(D)$ be the maximum number of internally disjoint and arc-disjoint $(S, r)$-trees in $D$, respectively. The generalized $k$-vertex-strong connectivity of $D$ is defined as $$κ_k(D)= \min \{κ_{S,r}(D)\…
A digraph is {\bf eulerian} if it is connected and every vertex has its in-degree equal to its out-degree. Having a spanning eulerian subdigraph is thus a weakening of having a hamiltonian cycle. A digraph is {\bf semicomplete} if it has no pair of non-adjacent vertices. A {\bf tournament} is a semicomplete digraph without directed cycles of length 2. Fraise and Thomassen \cite{fraisseGC3} proved that every $(k+1)$-strong tournament has a hamiltonian cycle which avoids any prescribed set of $k$ arcs. In \cite{bangsupereuler} the authors demonstrated that a number of results concerning vertex-c…
A digraph $G$ is \emph{$k$-geodetic} if for any pair of (not necessarily distinct) vertices $u,v \in V(G)$ there is at most one walk of length $\leq k$ from $u$ to $v$ in $G$. In this paper we determine the largest possible size of a $k$-geodetic digraph with given order. We then consider the more difficult problem of the largest size of a strongly-connected $k$-geodetic digraph with given order, solving this problem for $k = 2$ and giving a construction which we conjecture to be extremal for larger $k$. We close with some results on generalised Turán problems for the number of directed cycles…
On singular values and trace norm of signed digraphs
研究有向符号图邻接矩阵的奇异值，发现单圈和双圈有向图的奇异值在符号变化下保持不变，并利用奇异值的交错性质刻画秩为1的有向符号图，进而给出迹范数的下界及极图刻画。
Sharp bounds and extremal structure of minimally strongly $k$-connected digraphs
本文针对极小强k连通有向图，确定了其弧数的严格上界成立的最优阈值，证明了等号成立的充要条件，解决了Mader于1985年提出的相关猜想。
Antidirected forests in digraphs
本文证明每个弧数超过阈值 $g_k(n)$ 的有向图必包含任意给定 $k$ 条弧的反定向森林，并确定最大有向极值数为 $2\mathrm{ex}(n,kK_2)$，方法结合计数不等式、嵌入与分解。
Motivated by his work on the classification of countable homogeneous oriented graphs, Cherlin asked about the typical structure of oriented graphs (i) without a transitive triangle, or (ii) without an oriented triangle. We give an answer to these questions (which is not quite the predicted one). Our approach is based on the recent `hypergraph containers' method, developed independently by Saxton and Thomason as well as by Balogh, Morris and Samotij. Moreover, our results generalise to forbidden transitive tournaments and forbidden oriented cycles of any order, and also apply to digraphs. Along…
In $1967$, Vizing determined the maximum size of a graph with given order and radius. In $1973$, Fridman answered the same question for digraphs with given order and outradius. We investigate that question when restricting to biconnected digraphs. Biconnected digraphs are the digraphs with a finite total distance and hence the interesting ones, as we want to note a connection between minimizing the total distance and maximizing the size under the same constraints. We characterize the extremal digraphs maximizing the size among all biconnected digraphs of order $n$ and outradius $3$, as well as…
We say that $k$ graphs $G_1,G_2,\dots,G_k$ on a common vertex set of size $n$ contain a rainbow copy of a graph $H$ if their union contains a copy of $H$ with each edge belonging to a distinct $G_i$. We provide a counterexample to a conjecture of Frankl on the maximum product of the sizes of the edge sets of three graphs avoiding a rainbow triangle. We propose an alternative conjecture, which we prove under the additional assumption that the union of the three graphs is complete. Furthermore, we determine the maximum product of the sizes of the edge sets of three graphs or four graphs avoiding…
A directed graph is oriented if it can be obtained by orienting the edges of a simple, undirected graph. For an oriented graph $G$, let $β(G)$ denote the size of a minimum feedback arc set, a smallest subset of edges whose deletion leaves an acyclic subgraph. A simple consequence of a result of Berger and Shor is that any oriented graph $G$ with $m$ edges satisfies $β(G) = m/2 - Ω(m^{3/4})$. We observe that if an oriented graph $G$ has a fixed forbidden subgraph $B$, the upper bound of $β(G) = m/2 - Ω(m^{3/4})$ is best possible as a function of the number of edges if $B$ is not bipartite, but …
Sharp bounds for Laplacian spectral moments of digraphs with a fixed dichromatic number
本研究针对固定二色数的有向图，刻画了达到最小和最大拉普拉斯能量的图，并确定了联合有向图子类中第三拉普拉斯谱矩的尖锐界，将高阶情形留作开放问题。
Remoteness, order, size and connectivity constraints in digraphs
本文针对强连通有向图，给出了在给定阶、大小和顶点连通性条件下remoteness的尖锐上界，刻画了极值图，并将相关上界推广至欧拉有向图。
We refine previous results to provide examples, and in some cases precise classifications, of extremal subsets of {1,...,n} containing no solutions to a wide class of non-invariant, homogeneous linear equations in three variables, i.e.: equations of the form ax+by=cz with a+b \neq c.
Transversal Structures in Graph Systems: A Survey
本文综述了图系统中横截结构的极值问题，总结了确保存在横截结构的充分条件，并提出若干猜想和开放问题。
Extremal oriented graphs avoiding 1-subdivision of an in-star
该研究聚焦有向Turán数问题，针对入星1-细分图$\overrightarrow{S_{k,1}}$，确定了k=2、3时的精确极值有向图与最大弧数，并给出k≥4时该数的上下界。
Let $EG_r(n,k)$ denote the maximum number of edges in an $n$-vertex $r$-uniform hypergraph with no Berge cycles of length $k$ or longer. In the first part of this work, we have found exact values of $EG_r(n,k)$ and described the structure of extremal hypergraphs for the case when $k-2$ divides $n-1$ and $k\geq r+3$. In this paper we determine $EG_r(n,k)$ and describe the extremal hypergraphs for all $n$ when $k\geq r+4$.
Feedback edge set in bipartite digraph
研究二分有向图的反馈边集问题，证明4-自由二分有向图满足β(G)≤γ(G)/2，确定2k-自由强二分有向图的Turán数，并在k=2时证明更精确的界β(G)≤γ(G)/3。
Resolving a conjecture of Methuku and the first author we determine the size of the largest family of subsets of an $n$-element set avoiding both $Y_k$ and $Y_k'$ as induced subposets. The result follows as a consequence of the analogous result on a cyclical grid poset.
Stability of extremal connected hypergraphs avoiding Berge-paths
本文证明了避免 Berge-路径的连通一致超图极值结果的稳定性版本，通过构造 $\mathcal{H}_2$ 并证明超边数超过 $|\mathcal{H}_2|$ 的超图必为极值超图 $\mathcal{H}_1$ 的子超图。
On $T$-avoiding spherical codes and designs in $\mathbb{R}^{32}$
本文研究了$\mathbb{R}^{32}$中极值偶数单模格的最短向量作为$T$-避免最优解的性质，并探讨了其作为球面设计和码的应用。
Cut covers of acyclic digraphs
该研究证明无环有向图割覆盖的入度条件是紧的，给出有向路径幂图割覆盖的最大幂次范围，并证明相关同态有向图的割覆盖判定问题是NP完全的。
Given a graph $F$ and an integer $r \ge 2$, a partition $\widehat{F}$ of the edge set of $F$ into at most $r$ classes, and a graph $G$, define $c_{r, \widehat{F}}(G)$ as the number of $r$-colorings of the edges of $G$ that do not contain a copy of $F$ such that the edge partition induced by the coloring is isomorphic to the one of $F$. We think of $\widehat{F}$ as the pattern of coloring that should be avoided. The main question is, for a large enough $n$, to find the (extremal) graph $G$ on $n$ vertices which maximizes $c_{r, \widehat{F}}(G)$. This problem generalizes a question of Erd{\H o}s…
For positive integers $n$ and $r$, we consider $n$-vertex graphs with the maximum number of $r$-edge-colorings with no copy of a triangle where exactly two colors appear. We prove that, if $2 \leq r \leq 26$ and $n$ is sufficiently large, the maximum is attained by the bipartite Turán graph $T_2(n)$ on $n$ vertices. This is best possible, as $T_2(n)$ is not extremal for $r \geq 27$ colors and $n \geq 3$.
Spectrally symmetric orientations of graphs
本文研究了非二分图的谱对称定向图，探讨了其密度极值问题及构造方法。
Color-avoiding connected colorings and orientations
该研究刻画了边避色k-边连通图的特征，证明k=1时相关问题可多项式时间求解、k≥2时为NP难，还研究了对应定向问题及顶点着色、多色故障等推广情形，为网络鲁棒性分析提供理论支撑。
On $2$-connected graphs avoiding cycles of length $0$ modulo $4$
本文研究不含长度模4余0的圈的2-连通图的边数极值，证明了其至多有⌊(3n-1)/2⌋条边，并构造无穷多极值例子证明界是紧的。
Mutually avoiding Eulerian circuits
本文定义并研究双欧拉图，刻画允许两条从任意顶点出发且相互保持距离至少2的欧拉回路的图，并按边数确定最稠密与最稀疏情形。
$L_2$ Turán Problems for Small Tournaments and Stability
该研究聚焦小型锦标赛的$L_2$ Turán问题，确定了无$TT_4$、$R_4$等有向图的$L_2$范数平方最大值，补充了$Reg_5$的结果并证明了无$\boldsymbol{\text{的稳定性版本。
Sets avoiding a rainbow solution to the generalized Schur equation
本文研究避免广义Schur方程彩虹解的最大集合族，推广经典结果至多色情形，并确定所有极值结构。
Let k>=3 be an integer, we prove that the maximum induced density of the k-vertex directed star in a directed graph is attained by an iterated blow-up construction. This confirms a conjecture by Falgas-Ravry and Vaughan, who proved this for k=3, 4. This question provides the first known instance of density problem for which one can prove extremality of an iterated blow-up construction. We also study the inducibility of complete bipartite digraphs and discuss other related problems.
Two Arc-Disjoint Hamiltonian Paths in Finite Two-Generated Abelian Cayley Digraphs
证明了有限阿贝尔群上两个不同非零生成元的Cayley有向图存在两条弧不交哈密顿路径，通过切割反射定理和扇区填充不等式解决。
Let $D(n,r)$ be a random $r$-out regular directed multigraph on the set of vertices $\{1,\ldots,n\}$. In this work, we establish that for every $r \ge 2$, there exists $η_r>0$ such that $\text{diam}(D(n,r))=(1+η_r+o(1))\log_r{n}$. Our techniques also allow us to bound some extremal quantities related to the stationary distribution of a simple random walk on $D(n,r)$. In particular, we determine the asymptotic behaviour of $π_{\max}$ and $π_{\min}$, the maximum and the minimum values of the stationary distribution. We show that with high probability $π_{\max} = n^{-1+o(1)}$ and $π_{\min}=n^{-(1…
We show that sets avoiding 6-term arithmetic progressions in $\mathbb{Z}_6^n$ have size at most $5.709^n$. It is also pointed out that the "product construction" does not work in this setting, specially, we show that for the extremal sizes in small dimensions we have $r_6(\mathbb{Z}_6)=5$, $r_6(\mathbb{Z}_6^2)=25$ and $ 116\leq r_6(\mathbb{Z}_6^3)\leq 124$.
Avoiding configurations of small size in the square grid
本文研究了避免平方网格中特定几何配置的子集最大大小问题，结合概率方法与Sidon集性质，证明了避免菱形的下界，并探讨了kites避免问题的构造与影响。
The Exact End-Degree Threshold for Finite-Width Directed Hexagonal Grids
该研究解决了Hamann和Heuer提出的问题，确定了有限宽度有向六角网格的精确端度阈值k(n)，并得出对偶极值参数W(d)的表达式。
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