Johannes Carmesin
In 1972 Mader proved that every graph with average degree at least $4k$ has a $(k+1)$-connected subgraph with more than $2k$ vertices. We improve this bound by showing that the constant $4$ can be replaced by $3+\frac{1}{3}$; this bound is sharp.
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In 2009, Bang-Jensen asked whether there exists a function $g(k)$ such that every strongly $k$-connected $n$-vertex tournament contains a strongly $k$-connected spanning subgraph with at most $kn + g(k)$ arcs. In this paper, we answer the question by showing that every strongly $k$-connected $n$-vertex tournament contains a strongly $k$-connected spanning subgraph with at most $kn + 750k^2\log(k+1)$ arcs.
In this article the extremal structures for the mean order of connected induced subgraphs of cographs are determined. It is shown that among all connected cographs of order $n \ge 7$, the star $K_{1,n-1}$ has maximum mean connected induced subgraph order, and for $n \ge 3$, the $n$-skillet, $K_1+(K_1 \cup K_{n-2})$, has minimum mean connected induced subgraph order. It is deduced that the density for connected cographs (i.e. the ratio of the mean to the order of the graph) is asymptotically $1/2$. The mean order of all connected induced subgraphs containing a given vertex $v$ of a cograph $G$,…
We present an exact formula for the ordinary generating series of the simple paths between any two vertices of a graph. Our formula involves the adjacency matrix of the connected induced subgraphs and remains valid on weighted and directed graphs. As a particular case, we obtain a relation linking the Hamiltonian paths and cycles of a graph to its dominating connected sets.
Over all graphs (or unicyclic graphs) of a given order, we characterise those graphs that minimise or maximise the number of connected induced subgraphs. For each of these classes, we find that the graphs that minimise the number of connected induced subgraphs coincide with those that are known to maximise the Wiener index (the sum of the distances between all unordered pairs of vertices), and vice versa. For every $k$, we also determine the connected graphs that are extremal with respect to the number of $k$-vertex connected induced subgraphs. We show that, in contrast to the minimum which is…
Rigidity, arising in discrete geometry, is the property of a structure that does not flex. Laman provides a combinatorial characterization of rigid graphs in the Euclidean plane, and thus rigid graphs in the Euclidean plane have applications in graph theory. We discover a sufficient partition condition of packing spanning rigid subgraphs and spanning trees. As a corollary, we show that a simple graph $G$ contains a packing of $k$ spanning rigid subgraphs and $l$ spanning trees if $G$ is $(4k+2l)$-edge-connected, and $G-Z$ is essentially $(6k+2l - 2k|Z|)$-edge-connected for every $Z\subset V(G)…
The average order of the connected induced subgraphs of a graph $G$ is called the mean connected induced subgraph (CIS) order of $G$. This is an extension of the mean subtree order of a tree, first studied by Jamison. In this article, we demonstrate that among all connected block graphs of order $n$, the path $P_n$ has minimum mean CIS order. This extends a result of Jamison from trees to connected block graphs, and supports the conjecture of Kroeker, Mol, and Oellermann that $P_n$ has minimum mean CIS order among all connected graphs of order $n$.
In this paper, we consider the problem of decomposing the augmented cube $AQ_n$ into two spanning, regular, connected and pancyclic subgraphs. We prove that for $ n \geq 4$ and $ 2n - 1 = n_1 + n_2 $ with $ n_1, n_2 \geq 2,$ the augmented cube $ AQ_n$ can be decomposed into two spanning subgraphs $ H_1$ and $ H_2$ such that each $ H_i$ is $n_i$-regular and $n_i$-connected. Moreover, $H_i$ is $4$-pancyclic if $ n_i \geq 3.$
We characterise the structure of those graphs of a given order which maximise the number of connected induced subgraphs for seven different graph classes, each with other prescribed parameters like minimum degree, independence number, vertex cover number, vertex connectivity, edge connectivity, chromatic number, number of bridges, thereby contributing to filling a gap in the literature.
Let $G$ be a contraction critically quasi $5$-connected graph on at least $14$ vertices. If there is a vertex $x\in V_{4}(G)$ such that $G[N_{G}(x)]\cong K_{1,3}$ or $G[N_{G}(x)]\cong C_{4}$, then $G$ has a quasi $5$-contractible subgraph $H$ such that $0<\|V(H)\|<4$.
We consider the following question of Bollobas: given an r-colouring of the edges of the complete graph on n vertices, how large a k-connected subgraph can we find using only one colour? We solve this problem asymptotically when r-1 is a prime power, and exactly for 2- and 3-colourings.
A well-known result by Haxell and Kohayakawa states that the vertices of an $r$-coloured complete graph can be partitioned into $r$ monochromatic connected subgraphs of distinct colours; this is a slightly weaker variant of a conjecture by Erdős, Pyber and Gyárfás that states that there exists a partition into $r-1$ monochromatic connected subgraphs. We consider a variant of this problem, where the complete graph is replaced by a graph with large minimum degree, and prove two conjectures of Bal and DeBiasio, for two and three colours.
Extremal graphs with minimum number of connected subgraphs in a given family
本文定义了顶点的子图数，刻画了在 $n$ 个顶点和 $k$ 个割点的所有图中具有最少连通子图数的极值图及其顶点，并在围长至少为 $k$ 的条件下得出相应结果，最终完成了对 $0 \leq k \leq 4$ 情形的完整刻画。
In 1981, Duffus, Gould, and Jacobson showed that every connected graph either has a Hamiltonian path, or contains a claw ($K_{1,3}$) or a net (a fixed six-vertex graph) as an induced subgraph. This implies that subject to being connected, these two are the only minimal (under taking induced subgraphs) graphs with no Hamiltonian path. Brousek (1998) characterized the minimal graphs that are $2$-connected, non-Hamiltonian and do not contain the claw as an induced subgraph. We characterize the minimal graphs that are $2$-connected and non-Hamiltonian for two classes of graphs: (1) split graphs, (…
We investigate arrangements of hyperplanes whose normal vectors are given by connected subgraphs of a fixed graph. These include the resonance arrangement and certain ideal subarrangements of Weyl arrangements. We characterize those which are free, simplicial, factored, or supersolvable. In particular, such an arrangement is free if and only if the graph is a cycle, a path, an almost path, or a path with a triangle attached to it.
A vertex whose removal in a graph $G$ increases the number of components of $G$ is called a cut vertex. For all $n,c$, we determine the maximum number of connected induced subgraphs in a connected graph with order $n$ and $c$ cut vertices, and also characterise those graphs attaining the bound. Moreover, we show that the cycle has the smallest number of connected induced subgraphs among all cut vertex-free connected graphs. The general case $c>0$ remains an open task. We also characterise the extremal graph structures given both order and number of pendant vertices, and establish the correspon…
Let $G$ be a graph on $n$ vertices. An induced subgraph $H$ of $G$ is called heavy if there exist two nonadjacent vertices in $H$ with degree sum at least $n$ in $G$. We say that $G$ is $H$-heavy if every induced subgraph of $G$ isomorphic to $H$ is heavy. For a family $\mathcal{H}$ of graphs, $G$ is called $\mathcal{H}$-heavy if $G$ is $H$-heavy for every $H\in\mathcal{H}$. In this paper we characterize all connected graphs $R$ and $S$ other than $P_3$ (the path on three vertices) such that every 2-connected $\{R,S\}$-heavy graph is Hamiltonian. This extends several previous results on forbid…
Suppose the edges of the complete graph on n vertices are coloured using r colours; how large a k-connected subgraph are we guaranteed to find, which uses only at most s of the colours? This question is due to Bollobas, and the case s = 1 was considered in a preivous paper by the same authors. Here we consider the case s >= 2, and mention some open problems.
Bedrossian characterized all pairs of forbidden subgraphs for a 2-connected graph to be Hamiltonian. Instead of forbidding some induced subgraphs, we relax the conditions for graphs to be Hamiltonian by restricting Ore- and Fan-type degree conditions on these induced subgraphs. Let $G$ be a graph on $n$ vertices and $H$ be an induced subgraph of $G$. $H$ is called \emph{o}-heavy if there are two nonadjacent vertices in $H$ with degree sum at least $n$, and is called $f$-heavy if for every two vertices $u,v\in V(H)$, $d_{H}(u,v)=2$ implies that $\max\{d(u),d(v)\}\geq n/2$. We say that $G$ is $H…
Unavoidable Induced Subgraph of Infinite 2-connected Graphs
本文针对无限 2-连通图，推广 Ramsey 与 König 关于无限图不可避免诱导子图的经典定理，建立其 2-连通对应结果。
Sufficient average degree conditions for the existence of large highly connected subgraphs
针对Mader关于高度连通子图存在的平均度条件猜想，本文将已知最佳充分因子从低于3进一步改进至3.109，且约束所得$(k+1)$-连通子图顶点数超1.2k，还证明类似条件可存在更大的该类子图。
Note on extremal problems about connected subgraph sums
本文研究了图的连通子图和集合，以强形式证明了存在顶点赋值使得任意非同构图具有不同的连通子图和集合，并讨论了使所有连通子图和互异的顶点赋值。
Sufficient minimum degree conditions for the existence of highly connected or edge-connected subgraphs
研究图存在高连通或高边连通子图的最小度条件，证明了最小度至少为3k-1保证存在(k+1)-连通子图，以及平均度至少2k保证存在(k+1)-边连通子图等结果，并说明这些结果在某种意义下是紧的。
Bollobás and Gyárfás conjectured that for any $k, n \in \mathbb{Z}^+$ with $n > 4(k-1)$, every 2-edge-coloring of the complete graph on $n$ vertices leads to a $k$-connected monochromatic subgraph with at least $n-2k+2$ vertices. We find a counterexample with $n = \lfloor 5k-2.5-\sqrt{8k-\frac{31}{4}} \rfloor$, thus disproving the conjecture, and we show the conclusion holds for $n > 5k-2.5-\sqrt{8k-\frac{31}{4}}$ when $k \ge 16$.
Two-edge-connected (not necessarily spanning) subgraphs and polyhedra
研究两边连通子图多面体 TECSP(G)，用线性不等式刻画其格点并给出最大权子图 ILP 算法，同时分析面定义条件及实际效率。
We prove that any graph $G$ of minimum degree greater than $2k^2-1$ has a $(k+1)$-connected induced subgraph $H$ such that the number of vertices of $H$ that have neighbors outside of $H$ is at most $2k^2-1$. This generalizes a classical result of Mader, which states that a high minimum degree implies the existence of a highly connected subgraph. We give several variants of our result, and for each of these variants, we give asymptotics for the bounds. We also we compute optimal values for the case when $k=2$. Alon, Kleitman, Saks, Seymour, and Thomassen proved that in a graph of high chromati…
Spanning plane subgraphs of $1$-plane graphs
本文研究 $1$-平面图的生成平面子图，证明每个 $4$-连通 $1$-平面图都有连通生成平面子图，并给出不存在 $2$-连通此类子图的无穷反例，进而分析 $k$ 与 $l$ 的连通性条件。
A Sublinear Minimum-Degree Condition for $2$-Connected Subgraphs of All Orders
本文针对2-连通图，证明了亚线性最小度条件可保证其包含所有阶数从4到n的2-连通子图，填补了相关研究的空白。
Characterizing forbidden pairs for spanning $\varTheta$-subgraphs of 2-connected graphs
本文刻画使所有2-连通 $\{R,S\}$-free 图均含生成 $\varTheta$-子图的连通图禁用对，并刻画相应极小反例。
Imagine that a swarm of robots is given, these robots must communicate with each other, and they can do so if certain conditions are met. We say that the swarm is connected if there is at least one way to send a message between each pair of robots. A robot can move from a work station to another only if the connectivity of the swarm is preserved in order to perform some tasks. We model the problem via graph theory, we study connected subgraphs and how to motion them inside a connected graph preserving the connectivity. We determine completely the group of movements.
The path minimises the average size of a connected induced subgraph
本文证明在所有n阶图中，路径唯一最小化连通导出子图的平均阶数，证实了Kroeker-Mol-Oellermann猜想，推广了Jamison的树结果，并提供了更短的新证明。
On the sum of eccentric subgraph numbers of the vertices of a connected graph
针对连通图顶点的偏心子图数之和，定义了顶点的偏心子图数与图的偏心子图指数，研究了单圈图、树等特定图类中使偏心子图指数取最大值的图结构。
$k$-Connected Subgraphs of All Orders in Large Graphs with Minimum Degree at Least $n/q$
研究证实刘和宁猜想，即最小度至少为n/q的2-连通图含各阶2-连通子图。通过构造子图D及添加短路径，结合平均论证得到结果，还提出关于r-连通图的类似猜想。
Let $n, k, m$ be positive integers with $n\gg m\gg k$, and let $\mathcal{A}$ be the set of graphs $G$ of order at least 3 such that there is a $k$-connected monochromatic subgraph of order at least $n-f(G,k,m)$ in any rainbow $G$-free coloring of $K_n$ using all the $m$ colors. In this paper, we prove that the set $\mathcal{A}$ consists of precisely $P_6$, $P_3\cup P_4$, $K_2\cup P_5$, $K_2\cup 2P_3$, $2K_2\cup K_3$, $2K_2\cup P^{+}_4$, $3K_2\cup K_{1,3}$ and their subgraphs of order at least 3. Moreover, we show that for any graph $H\in \mathcal{A}$, if $n$ sufficiently larger than $m$ and $k…
A graph on 5 vertices consisting of 2 copies of the cycle graph C3 sharing a common vertex is called the Butterfly graph (B). The smallest natural number s such that any two-colouring (say red and blue) of the edges of Kj*s has a copy of a red B or a blue G is called the multipartite Ramsey number of Butterfly graph versus G. This number is denoted by mj(B,G). In this paper we find exact the values for mj(B,G) when and G represents any connected proper subgraph of K4 with at least one edge.
Let $G$ be a graph and let $l$ be an integer-valued function on subsets of $V(G)$. The graph $G$ is said to be $l$-partition-connected, if for every partition $P$ of $V(G)$, $e_G(P)\ge \sum_{A\in P} l(A)-l(V(G))$, where $e_G(P)$ denotes the number of edges of $G$ joining different parts of $P$. We say that $G$ is $l$-rigid, if it contains a spanning $l$-partition-connected subgraph $H$ with $|E(H)|=\sum_{v\in V(H)} l(v)-l(V(H))$. In this paper, we investigate decomposition of graphs into spanning partition-connected and spanning rigid subgraphs. As a consequence, we improve a recent result due…
We classify connected spanning convex subgraphs of the square cycles. We then show that every spanning tree of $C_n^2$ is contained in a unique nontrivial connected spanning convex subgraph of $C_n^2$. As a result, we obtain a purely combinatorial derivation of the formula for the number of spanning trees of the square cycles.
Unavoidable substructures in large and infinite $2$-edge-connected graphs
本文研究了大且无限2-边连通图中的不可避免诱导子结构，发现'pinched super-clean ladders'等结构在其中扮演关键角色，并推导出关于大子图、拓扑Minor、Minor、诱导拓扑Minor、诱导Minor及欧拉子图的相关结论。
Free multiderivations of connected subgraph arrangements
本文将 Cuntz–Kühne 关于连通子图配置自由性的图分类推广到多重配置，刻画了存在重数 μ 使 (A_G, μ) 自由的全部图 G。
Highly edge-connected regular graphs without large factorizable subgraphs
本文构造了高度边连通的 r-正则图，证明其中不存在 r-2 个两两不交的完美匹配，部分回答了 Thomassen 关于正则图因子分解的问题。
Mader proved that for $k\geq 2$ and $n\geq 2k$, every $n$-vertex graph with no $(k+1)$-connected subgraphs has at most $(1+\frac{1}{\sqrt{2}})k(n-k)$ edges. He also conjectured that for $n$ large with respect to $k$, every such graph has at most $\frac{3}{2}\left(k - \frac{1}{3}\right)(n-k)$ edges. Yuster improved Mader's upper bound to $\frac{193}{120}k(n-k)$ for $n\geq\frac{9k}{4}$. In this note, we make the next step towards Mader's Conjecture: we improve Yuster's bound to $\frac{19}{12}k(n-k)$ for $n\geq\frac{5k}{2}$.
Highly connected orientations from edge-disjoint rigid subgraphs
本文肯定回答了 Thomassen 猜想，证明 $O(k^2)$ 阶连通度足以保证图具有 $k$-顶点连通定向，并利用边不交刚性子图工具同时解决了 Kriesell 关于生成树补图连通性的猜想。
The three subgraphs of a connected graph induced by the center, annulus and periphery are called its metric subgraphs. The main results are as follows. (1) There exists a graph of order $n$ whose metric subgraphs are all paths if and only if $n\ge 13$ and the smallest size of such a graph of order $13$ is $22;$ (2) there exists a graph of order $n$ whose metric subgraphs are all cycles if and only if $n\ge 15,$ and there are exactly three such graphs of order $15;$ (3) for every integer $k\ge 3,$ we determine the possible orders for the existence of a graph whose metric subgraphs are all conne…
Given $k\ge 1$, a $k$-proper partition of a graph $G$ is a partition ${\mathcal P}$ of $V(G)$ such that each part $P$ of ${\mathcal P}$ induces a $k$-connected subgraph of $G$. We prove that if $G$ is a graph of order $n$ such that $δ(G)\ge \sqrt{n}$, then $G$ has a $2$-proper partition with at most $n/δ(G)$ parts. The bounds on the number of parts and the minimum degree are both best possible. We then prove that If $G$ is a graph of order $n$ with minimum degree $δ(G)\ge\sqrt{c(k-1)n}$, where $c=\frac{2123}{180}$, then $G$ has a $k$-proper partition into at most $\frac{cn}{δ(G)}$ parts. This …
Let $F(G)$ be the number of forests of a graph $G$. Similarly let $C(G)$ be the number of connected spanning subgraphs of a connected graph $G$. We bound $F(G)$ and $C(G)$ for regular graphs and for graphs with fixed average degree. Among many other things we study $f_d=\sup_{G\in \mathcal{G}_d}F(G)^{1/v(G)}$, where $\mathcal{G}_d$ is the family of $d$--regular graphs, and $v(G)$ denotes the number of vertices of a graph $G$. We show that $f_3=2^{3/2}$, and if $(G_n)_n$ is a sequence of $3$--regular graphs with length of the shortest cycle tending to infinity, then $\lim_{n\to \infty}F(G_n)^{1…
Unavoidable Induced Subgraphs of Large 2-Connected Graphs
本文针对2-连通图拓展Ramsey定理，证明足够大的2-连通图必包含指定的几类诱导子图之一，完善了图论中不可避免诱导子图的相关结论。
In 1974, Goodman and Hedetniemi proved that every 2-connected $(K_{1,3},K_{1,3}+e)$-free graph is hamiltonian. This result gave rise many other hamiltonicity conditions for various pairs and triples of forbidden connected subgraphs under additional connectivity conditions. In 1997, it was proved that a single forbidden connected subgraph $R$ in 2-connected graphs can create only a trivial class of hamiltonian graphs (complete graphs) with $R=P_3$. In this paper we prove that a single forbidden subgraph $R$ can create a non trivial class of hamiltonian graphs if $R$ is disconnected: $(\ast1)$ e…
Given a graph $G$ and a graph property $P$ we say that $G$ is minimal with respect to $P$ if no proper induced subgraph of $G$ has the property $P$. An HC-obstruction is a minimal 2-connected non-Hamiltonian graph. Given a graph $H$, a graph $G$ is $H$-free if $G$ has no induced subgraph isomorphic to $H$. The main motivation for this paper originates from a theorem of Duffus, Gould, and Jacobson (1981), which characterizes all the minimal connected graphs with no Hamiltonian path. In 1998, Brousek characterized all the claw-free HC-obstructions. On a similar note, Chiba and Furuya (2021), cha…
Color-avoiding connected spanning subgraphs with minimum number of edges
本文研究避色连通图的最大可移除边数问题，证明其为NP难问题，提出多项式时间近似算法，确定了给定参数下避色连通图的最小边数，并将边避色连通性推广至拟阵。
We present a $\frac{10}{7}$-approximation algorithm for the minimum two-vertex-connected spanning subgraph problem.
An Improved Interpolation Theorem and Disproofs of Two Conjectures on 2-Connected Subgraphs
本文改进了2-连通子图的插值定理，并驳斥了两个关于2-连通子图的猜想，同时提出了新的猜想。
A graph $G$ on $n$ vertices is Hamiltonian if it contains a spanning cycle, and pancyclic if it contains cycles of all lengths from 3 to $n$. In 1984, Fan presented a degree condition involving every pair of vertices at distance two for a 2-connected graph to be Hamiltonian. Motivated by Fan's result, we say that an induced subgraph $H$ of $G$ is $f_1$-heavy if for every pair of vertices $u,v\in V(H)$, $d_{H}(u,v)=2$ implies $\max\{d(u),d(v)\}\geq (n+1)/2$. For a given graph $R$, $G$ is called $R$-$f_1$-heavy if every induced subgraph of $G$ isomorphic to $R$ is $f_1$-heavy. In this paper we s…
The minimum size of a $3$-connected locally nonforesty graph
本研究针对Chernyshev等人提出的3-连通局部非森林图规模下界猜想，通过确定n阶3-连通局部非森林图的最小规模，证明该猜想不成立。
The minimum size of a $k$-connected locally nonforesty graph
本文研究了局部非森林图（每个顶点的邻域诱导子图含圈）的性质，并确定了阶为$n$的$k$-连通局部非森林图的最小边数。
We study optimal minimum degree conditions when an $n$-vertex graph $G$ contains an $r$-regular $r$-connected subgraph. We prove for $r$ fixed and $n$ large the condition to be $δ(G) \ge \frac{n+r-2}{2}$ when $nr \equiv 0 \pmod 2$. This answers a question of M.~Kriesell.
We prove that every (6k + 2l, 2k)-connected simple graph contains k rigid and l connected edge-disjoint spanning subgraphs. This implies a theorem of Jackson and Jordán [4] and a theorem of Jordán [6] on packing of rigid spanning subgraphs. Both these results are generalizations of the classical result of Lovász and Yemini [9] saying that every 6-connected graph is rigid for which our approach provides a transparent proof. Our result also gives two improved upper bounds on the connectivity of graphs that have interesting properties: (1) every 8-connected graph packs a spanning tree and a 2-con…
A path in a vertex-colored graph is called \emph{vertex-rainbow} if its internal vertices have pairwise distinct colors. A graph $G$ is \emph{rainbow vertex-connected} if for any two distinct vertices of $G$, there is a vertex-rainbow path connecting them. For a connected graph $G$, the \emph{rainbow vertex-connection number} of $G$, denoted by $rvc(G)$, is defined as the minimum number of colors that are required to make $G$ rainbow vertex-connected. In this paper, we find all the families $\mathcal{F}$ of connected graphs with $|\mathcal{F}|\in\{1,2\}$, for which there is a constant $k_\math…
The subgraph eigenvector centrality of graphs
本文提出基于子图结构的特征向量中心性，利用子图张量刻画顶点重要性，并证明其存在性，能区分正则图中经典中心性无法区分的顶点。
We give a randomized $1+\frac{5.06}{\sqrt{k}}$-approximation algorithm for the minimum $k$-edge connected spanning multi-subgraph problem, $k$-ECSM.
A graph is $1$-planar if it has a drawing in the plane such that each edge is crossed at most once by another edge. Moreover, if this drawing has the additional property that for each crossing of two edges the end vertices of these edges induce a complete subgraph, then the graph is locally maximal $1$-planar. For a $3$-connected locally maximal $1$-planar graph $G$, we show the existence of a spanning $3$-connected planar subgraph and prove that $G$ is hamiltonian if $G$ has at most three $3$-vertex-cuts, and that $G$ is traceable if $G$ has at most four $3$-vertex-cuts. Moreover, infinitely …
Recognition Complexity of Subgraphs of k-Connected Planar Cubic Graphs
本文研究了k=1,2,3时k-连通平面三次图子图的识别复杂性，提出了识别1-和2-连通平面三次图子图的多项式时间算法，并证明了在可变嵌入设置下识别3-连通平面三次图子图是NP困难的。
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