James Davies
We prove that there are intersection graphs of axis-aligned boxes in $\mathbb{R}^3$ and intersection graphs of straight lines in $\mathbb{R}^3$ that have arbitrarily large girth and chromatic number.
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Erdős proved that there are graphs with arbitrarily large girth and chromatic number. We study the extension of this for generalized chromatic numbers.
In this short note, we extend the result of Galluccio, Goddyn, and Hell, which states that graphs of large girth excluding a minor are nearly bipartite. We also prove a similar result for the oriented chromatic number, from which follows in particular that graphs of large girth excluding a minor have oriented chromatic number at most $5$, and for the $p$th chromatic number $χ_p$, from which follows in particular that graphs $G$ of large girth excluding a minor have $χ_p(G)\leq p+2$.
This paper settled the question, which remains open in article published in Discrete Mathematics in 2008. Graphs homeomorphic to $K_4$, i.e., cliques on 4 vertices with edges replaced by paths are considered in this work. This work completes the study of chromaticity of $K_4$-homeomorphs of girth 7.
A class of simple graphs such as ${\cal G}$ is said to be {\it odd-girth-closed} if for any positive integer $g$ there exists a graph $G \in {\cal G}$ such that the odd-girth of $G$ is greater than or equal to $g$. An odd-girth-closed class of graphs ${\cal G}$ is said to be {\it odd-pentagonal} if there exists a positive integer $g^*$ depending on ${\cal G}$ such that any graph $G \in {\cal G}$ whose odd-girth is greater than $g^*$ admits a homomorphism to the five cycle (i.e. is $C_{_{5}}$-colorable). In this article, we show that finding the odd girth of generalized Petersen graphs can be t…
On the solution to the Erdős-Hajnal problem on high-girth high-chromatic subgraphs
本文解释并优化了 Erdős-Hajnal 问题的高围长高色数子图的否定构造，将 Kohlmeyer 和 Kruer 的色数上界从 6 改进到 3。
The distinguishing number $D(G)$ of a graph $G$ is the least integer $d$ such that $G$ has a vertex labeling with $d$ labels that is preserved only by a trivial automorphism. The distinguishing chromatic number $χ_{D}(G)$ of $G$ is defined similarly, where, in addition, $f$ is assumed to be a proper labeling. Motivated by a conjecture in \cite{colins}, we prove that if $G$ is a bipartite graph of girth at least six with the maximum degree $Δ(G)$, then $χ_{D}(G)\leq Δ(G)+1$. We also obtain an upper bound for $χ_{D}(G)$ where $G$ is a graph with at most one cycle. Finally, we state a relationshi…
Unicyclic Graphs of Arbitrary Girth with the Same Chromatic Symmetric Function
研究图的色对称函数能否区分非同构单圈图，通过应用“三重删除”模关系，展示了一系列围长递增且具有相同色对称函数的非同构连通单圈图对，还给出首个具有相同色对称函数的无限二分图族。
We introduce a new method for computing bounds on the independence number and fractional chromatic number of classes of graphs with local constraints, and apply this method in various scenarios. We establish a formula that generates a general upper bound for the fractional chromatic number of triangle-free graphs of maximum degree~$Δ\ge 3$. This upper bound matches that deduced from the fractional version of Reed's bound for small values of~$Δ$, and improves it when~$Δ\ge 17$, transitioning smoothly to the best possible asymptotic regime, barring a breakthrough in Ramsey theory. Focusing on sm…
A strong $k$-edge-coloring of a graph $G$ is a mapping from $E(G)$ to $\{1,2,\ldots,k\}$ such that every pair of distinct edges at distance at most two receive different colors. The strong chromatic index $χ'_s(G)$ of a graph $G$ is the minimum $k$ for which $G$ has a strong $k$-edge-coloring. Denote $σ(G)=\max_{xy\in E(G)}\{\operatorname{deg}(x)+\operatorname{deg}(y)-1\}$. It is easy to see that $σ(G) \le χ'_s(G)$ for any graph $G$, and the equality holds when $G$ is a tree. For a planar graph $G$ of maximum degree $Δ$, it was proved that $χ'_s(G) \le 4 Δ+4$ by using the Four Color Theorem. T…
Reed conjectured that for every epsilon>0 and Delta there exists g such that the fractional total chromatic number of a graph with maximum degree Delta and girth at least g is at most Delta+1+epsilon. We prove the conjecture for Delta=3 and for even Delta>=4 in the following stronger form: For each of these values of Delta, there exists g such that the fractional total chromatic number of any graph with maximum degree Delta and girth at least g is equal to Delta+1.
A b-coloring of a graph is a coloring of its vertices such that every color class contains a vertex that has a neighbor in all other classes. The b-chromatic number of a graph is the largest integer k such that the graph has a b-coloring with k colors. We show how to compute in polynomial time the b-chromatic number of a graph of girth at least 9. This improves the seminal result of Irving and Manlove on trees.
Brooks' Theorem states that a connected graph $G$ of maximum degree $Δ$ has chromatic number at most $Δ$, unless $G$ is an odd cycle or a complete graph. A result of Johansson (1996) shows that if $G$ is triangle-free, then the chromatic number drops to $O(Δ/ \log Δ)$. In this paper, we derive a weak analog for the chromatic number of digraphs. We show that every (loopless) digraph $D$ without directed cycles of length two has chromatic number $χ(D) \leq (1-e^{-13}) \tildeΔ$, where $\tildeΔ$ is the maximum geometric mean of the out-degree and in-degree of a vertex in $D$, when $\tildeΔ$ is suf…
In this article we consider a problem related to two famous combinatorial topics. One of them concerns the chromatic number of the space. The other deals with graphs having big girth (the length of the shortest cycle) and large chromatic number. Namely, we prove that for any $l\in \mathbb{N}$ there exists a sequence of distance graphs in $\mathbb{R}^n$ with girth at least $l$ and the chromatic number equal to $(c+\bar{o}(1))^n$ with $c>1$.
We study the computational efficiency of approaches, based on Hilbert's Nullstellensatz, which use systems of linear equations for detecting non-colorability of graphs having large girth and chromatic number. We show that for every non-$k$-colorable graph with $n$ vertices and girth $g>4k$, the algorithm is required to solve systems of size at least $n^{Ω(g)}$ in order to detect its non-$k$-colorability.
On the zero-free region for the chromatic polynomial of graphs with maximum degree $Δ$ and girth $g$
本文为最大度为 $Δ$ 且围长为 $g$ 的图提供了色多项式 $P_G(q)$ 在复平面上的无零点区域界限，在 $Δ$ 较小时显著改进了已有结果。
Defective and Clustered Colouring of Graphs with Given Girth
本文研究给定围长图在特定minor-闭类中的缺陷染色与聚类染色问题，确定或建立其色数界，发现无三角形且树宽为$k$的图类的三种色数相等，结果可推广至树深为$k$及无$K_p$子图的图类。
We give an upper bound for the online chromatic number of graphs with high girth and for graphs with high oddgirth generalizing Kier- stead's algorithm for graphs that contain neither a C3 or C5 as an induced subgraph.
It is known (Bollobás (1978); Kostochka and Mazurova (1977)) that there exist graphs of maximum degree $Δ$ and of arbitrarily large girth whose chromatic number is at least $c Δ/ \log Δ$. We show an analogous result for digraphs where the chromatic number of a digraph $D$ is defined as the minimum integer $k$ so that $V(D)$ can be partitioned into $k$ acyclic sets, and the girth is the length of the shortest cycle in the corresponding undirected graph. It is also shown, in the same vein as an old result of Erdos (1962), that there are digraphs with arbitrarily large chromatic number where ever…
We show that the circular chromatic index of a (sub)cubic graph with girth at least six is at most 7/2.
Geometric graphs with exponential chromatic number and arbitrary girth
本文证明了在$\mathbb{R}^d$中存在具有任意大围长且色数至少为$(1.074 + o(1))^d$的单位距离图，改进了已有结果并给出了首个指数底数不随围长趋向1的界限，该结论可推广至其他几何图。
We utilize relations between Khovanov and chromatic graph homology to determine extreme Khovanov groups and corresponding coefficients of the Jones polynomial. The extent to which chromatic homology and chromatic polynomial can be used to compute integral Khovanov homology of a link depends on the maximal girth of its all-positive graphs. In this paper we also define the girth of a link, discuss relations to other knot invariants, and the possible values for girth. Analyzing girth leads to a description of possible all-A state graphs of any given link; e.g., if a link has a diagram such that t…
We prove that for every $k$ and every $\varepsilon>0$, there exists $g$ such that every graph with tree-width at most $k$ and odd-girth at least $g$ has circular chromatic number at most $2+\varepsilon$.
For every positive integer $n$, we construct a Hasse diagram with $n$ vertices and chromatic number $Ω(n^{1/4})$, which significantly improves on the previously known best constructions of Hasse diagrams having chromatic number $Θ(\log n)$. In addition, if we also require that our Hasse diagram has girth at least $k\geq 5$, we can achieve a chromatic number of at least $n^{\frac{1}{2k-3}+o(1)}$. These results have the following surprising geometric consequence. They imply the existence of a family $\mathcal{C}$ of $n$ curves in the plane such that the disjointness graph $G$ of $\mathcal{C}$ is…
Tight factorizations of girth-$g$-regular graphs
研究围长、正则度和色指数相等的围长正则图，确定每个1-因子均与所有围长圈相交的1-因子分解，并探讨其在哈密顿可分解性、三维几何及优化问题中的应用。
On conflict-free colorings of cyclic polytopes and the girth conjecture for graphs
本文研究循环多面体对应超图的无冲突色数，给出若干偶数维的精确渐近界及一般偶数维上下界，并揭示其与 Erdős 围长猜想的联系。
We show that every (sub)cubic n-vertex graph with sufficiently large girth has fractional chromatic number at most 2.2978 which implies that it contains an independent set of size at least 0.4352n. Our bound on the independence number is valid to random cubic graphs as well as it improves existing lower bounds on the maximum cut in cubic graphs with large girth.
An injective coloring of a graph is a vertex coloring where two vertices with common neighbor receive distinct colors. The minimum integer $k$ that $G$ has a $k-$injective coloring is called injective chromatic number of $G$ and denoted by $χ_i(G)$. In this paper, the injective chromatic number of outerplanar graphs with maximum degree $Δ$ and girth $g$ is studied. It is shown that for every outerplanar graph, $χ_i(G)\leq Δ+2$, and this bound is tight. Then, it is proved that for outerplanar graphs with $Δ=3$, $χ_i(G)\leq Δ+1$ and the bound is tight for outerplanar graphs of girth three and $4…
Triangle-free subgraphs with large fractional chromatic number
本文证明了 Erdős-Hajnal 猜想中围长为4情形的分数色数版本，即存在函数 h(k,4)，使色数足够大的图必含分数色数至少为 k 的无三角形子图。
Graphs of large girth
本文是一篇关于大围长图的综述，分为两部分：第一部分探讨与Moore bound相关的代数与极值图论，第二部分从大色数与大围长图构造出发讨论girth Ramsey theorem，两部分可独立阅读。
The strong chromatic index of a graph $G$, denoted $χ_s'(G)$, is the least number of colors needed to edge-color $G$ so that edges at distance at most two receive distinct colors. The strong list chromatic index, denoted $χ_{s,\ell}'(G)$, is the least integer $k$ such that if arbitrary lists of size $k$ are assigned to each edge then $G$ can be edge-colored from those lists where edges at distance at most two receive distinct colors. We use the discharging method, the Combinatorial Nullstellensatz, and computation to show that if $G$ is a subcubic planar graph with $\operatorname{girth}(G) \ge…
We prove that every triangle-free graph with maximum degree $Δ$ has list chromatic number at most $(1+o(1))\fracΔ{\ln Δ}$. This matches the best-known bound for graphs of girth at least 5. We also provide a new proof that for any $r\geq 4$ every $K_r$-free graph has list-chromatic number at most $200r\frac{Δ\ln\lnΔ}{\lnΔ}$.
A graph with chromatic number $k$ is called $k$-chromatic. Using computational methods, we show that the smallest triangle-free 6-chromatic graphs have at least 32 and at most 40 vertices. We also determine the complete set of all triangle-free 5-chromatic graphs up to 24 vertices. This implies that Reed's conjecture holds for triangle-free graphs up to at least this order. We also establish that the smallest regular triangle-free 5-chromatic graphs have 24 vertices. Finally, we show that the smallest 5-chromatic graphs of girth at least 5 have at least 29 vertices and that the smallest 4-chro…
We consider two graph colouring problems in which edges at distance at most $t$ are given distinct colours, for some fixed positive integer $t$. We obtain two upper bounds for the distance-$t$ chromatic index, the least number of colours necessary for such a colouring. One is a bound of $(2-\eps)Δ^t$ for graphs of maximum degree at most $Δ$, where $\eps$ is some absolute positive constant independent of $t$. The other is a bound of $O(Δ^t/\log Δ)$ (as $Δ\to\infty$) for graphs of maximum degree at most $Δ$ and girth at least $2t+1$. The first bound is an analogue of Molloy and Reed's bound on t…
Bounds for the smallest $k$-chromatic graphs of given girth
本文研究围长至少为 $g$ 的 $k$-色图的最小阶数 $n_g(k)$，通过穷举搜索和计算机算法给出新的上下界，并首次提供 $k=4$、$g>5$ 的小阶数实例。
On colouring oriented graphs of large girth
该研究证明任意有向图都存在满足大围长、同态存在性等价及唯一分解性质的满同态原像图，由此推导有向图围长与有向色数的关联，并给出特定参数下的构造性结果。
The distinguishing chromatic number, $χ_D(G)$, of a graph $G$ is the smallest number of colors in a proper coloring, $φ$, of $G$, such that the only automorphism of $G$ that preserves all colors of $φ$ is the identity map. Collins and Trenk conjectured that if $G$ is connected with girth at least 5 and $G\ne C_6$, then $χ_D(G)\le Δ+1$. We prove this conjecture.
Improved bounds for the smallest 4-chromatic graph of girth six
本文将围长为6的最小4-色图的阶数界改进为29≤n₆(4)≤64，构造出64顶点的见证图并通过形式化验证，同时给出结构障碍结果。
Digraphs of Large Girth and Dichromatic Number in Tournaments with Large Dichromatic Number
本文证明每个二色数足够大的竞赛图包含具有大有向围长和大二色数的子有向图，并探讨了用底层图围长替代的情形，该情形蕴含Erdős-Hajnal猜想且对特定竞赛图族成立。
Sharp bounds for the fractional chromatic number of high-girth $d$-degenerate graphs
本文证实了高围长d-退化图分数色数的上界猜想，给出高效随机算法，还建立了更强形式的下界，揭示均匀附着模型无Erdős-Rényi图中的算法间隙。
We prove that in sparse Erdős-Rényi graphs of average degree $d$, the vector chromatic number (the relaxation of chromatic number coming from the Lovàsz theta function) is typically $\tfrac{1}{2}\sqrt{d} + o_d(1)$. This fits with a long-standing conjecture that various refutation and hypothesis-testing problems concerning $k$-colorings of sparse Erdős-Rényi graphs become computationally intractable below the `Kesten-Stigum threshold' $d_{KS,k} = (k-1)^2$. Along the way, we use the celebrated Ihara-Bass identity and a carefully constructed non-backtracking random walk to prove two deterministic…
A classical result of Erdős and Hajnal claims that for any integers $k, r, g \geq 2$ there is an $r$-uniform hypergraph of girth at least $g$ with chromatic number at least $k$. This implies that there are sparse hypergraphs such that in any coloring of their vertices with at most $k-1$ colors there is a monochromatic hyperedge. We show that for any integers $r, g\geq 2$ there is an $r$-uniform hypergraph of girth at least $g$ such that in any coloring of its vertices there is either a monochromatic or a rainbow (totally multicolored) edge. We give a probabilistic and a deterministic proof of …
We prove that for all nonnegative integers k,s there exists c with the following property. Let G be a graph with clique number at most k and chromatic number more than c. Then for every vertex-colouring (not necessarily optimal) of G, some induced subgraph of G is an s-vertex path, and all its vertices have different colours. This extends a recent result of Gyarfas and Sarkozy, who proved the same (when k=2) for graphs G with girth at least five.
A b-coloring of a graph is a proper coloring of its vertices such that each color class contains a vertex adjacent to at least one vertex of every other color class. The b-chromatic number of a graph is the largest integer k such that the graph has a b-coloring with k colors. In this work we present lower bounds for the b-chromatic number of a vertex-deleted subgraph of a graph, particularly regarding two important classes, quasi-line and chordal graphs. We also get bounds for the b-chromatic number of G -{x}, when G is a graph with large girth.
In this work, the classical Nelson -- Hadwiger problem is studied which lies on the edge of combinatorial geometry and graph theory. It concerns colorings of distance graphs in $ {\mathbb R}^n $, i.e., graphs such that their vertices are vectors and their edges are pairs of vectors at a distance from a given set of postive numbers apart. A series of new lower bounds are obtained for the chromatic numbers of such graphs with different restrictions on the clique numbers and the girths.
We prove that for any graph $G$ of maximum degree at most $Δ$, the zeros of its chromatic polynomial $χ_G(z)$ (in $\mathbb{C}$) lie outside the disk of radius $5.02 Δ$ centered at $0$. This improves on the previously best known bound of approximately $6.91Δ$. In the case of graphs of high girth we can improve this. We prove that for every $g$ there is a constant $K_g$ such that for any graph $G$ of maximum degree at most $Δ$ and girth at least $g$, the zeros of its chromatic polynomial $χ_G(z)$ lie outside the disk of radius $K_g Δ$ centered at $0$ where $K_g \to 1 + e \approx 3.72$ as $g \to …
We denote by $χ$ g (G) the game chromatic number of a graph G, which is the smallest number of colors Alice needs to win the coloring game on G. We know from Montassier et al. [M. Montassier, P. Ossona de Mendez, A. Raspaud and X. Zhu, Decomposing a graph into forests, J. Graph Theory Ser. B, 102(1):38-52, 2012] and, independantly, from Wang and Zhang, [Y. Wang and Q. Zhang. Decomposing a planar graph with girth at least 8 into a forest and a matching, Discrete Maths, 311:844-849, 2011] that planar graphs with girth at least 8 have game chromatic number at most 5. One can ask if this bound of …
The $b$-chromatic number of a graph $G$, denoted by $b(G)$, is the largest positive integer $k$ such that there exists a proper coloring for G with $k$ colors in which every color class contains at least one vertex adjacent to some vertex in each of the other color classes, such a vertex is called a dominant vertex. The $f$-chromatic vertex number of a $d$-regular graph $G$, denoted by $f(G)$, is the maximum number of dominant vertices of distinct colors in a proper coloring with $d+1$ colors. El Sahili and Kouider conjectured that $b(G)=d+1$ for any $d$-regular graph $G$ of girth 5. We study …
Shift Graphs, Chromatic Number and Acyclic One-Path Orientations
本文利用移位图及其诱导子图，改进了完全二分图情形下无F诱导子图的色数上界，并证明移位图无AOP性质，同时构造了高奇围长但无AOP性质的图。
On ${\rm b}^{\ast}$-Coloring and $z$-Coloring of graphs with high girth
该研究针对高围长图，运用列表着色技术推导了${\rm b}^*$-色数、$z$-色数的相关结论，丰富了图着色理论的成果。
We present a necessary and sufficient condition for a graph of odd-girth $2k+1$ to bound the class of $K_4$-minor-free graphs of odd-girth (at least) $2k+1$, that is, to admit a homomorphism from any such $K_4$-minor-free graph. This yields a polynomial-time algorithm to recognize such bounds. Using this condition, we first prove that every $K_4$-minor free graph of odd-girth $2k+1$ admits a homomorphism to the projective hypercube of dimension $2k$. This supports a conjecture of the third author which generalizes the four-color theorem and relates to several outstanding conjectures such as Se…
A graph G is (a:b)-colorable if there exists an assignment of b-element subsets of {1,...,a} to vertices of G such that sets assigned to adjacent vertices are disjoint. We first show that for every triangle-free planar graph G and a vertex x of G, the graph G has a set coloring phi by subsets of {1,...,6} such that |phi(v)|>=2 for each vertex v of G and |phi(x)|=3. As a corollary, every triangle-free planar graph on n vertices is (6n:2n+1)-colorable. We further use this result to prove that for every Delta, there exists a constant M_Delta such that every planar graph G of girth at least five a…
The Additive Coloring Problem is a variation of the Coloring Problem where labels of $\{1,\ldots,k\}$ are assigned to the vertices of a graph $G$ so that the sum of labels over the neighborhood of each vertex is a proper coloring of $G$. The least value $k$ for which $G$ admits such labeling is called \emph{additive chromatic number} of $G$. This problem was first presented by Czerwiński, Grytczuk and Żelazny who also proposed a conjecture that for every graph $G$, the additive chromatic number never exceeds the classic chromatic number. Up to date, the conjecture has been proved for complete …
$\mathcal{O}(VE)$ time algorithms for the Grundy (First-Fit) chromatic number of block graphs and graphs with sufficiently large girth
针对图的Grundy色数NP完全且难以近似的问题，提出确定块图及充分大围长图Grundy数的$\mathcal{O}(VE)$时间精确算法，并为含割点图提供上界及一般图的多项式时间近似算法。
An improved bound for 2-distance coloring of planar graphs with girth six
针对围长至少为6且最大度$Δ\neq6$的平面图，证明其2-距离色数$χ_2(G)\neqΔ+4$，改进了该类图2-距离着色的已知最优上界。
A packing $k$-coloring of a graph $G$ is a partition of $V(G)$ into sets $V_1,\ldots,V_k$ such that for each $1\leq i\leq k$ the distance between any two distinct $x,y\in V_i$ is at least $i+1$. The packing chromatic number, $χ_p(G)$, of a graph $G$ is the minimum $k$ such that $G$ has a packing $k$-coloring. Sloper showed that there are $4$-regular graphs with arbitrarily large packing chromatic number. The question whether the packing chromatic number of subcubic graphs is bounded appears in several papers. We answer this question in the negative. Moreover, we show that for every fixed $k$ a…
Locally bipartite subgraphs via multicolor Ramsey numbers
该研究针对Erdős-Hajnal猜想，改进了$f_4(k)$的上界，还证明了奇数$g\ge5$时对应函数$h_g(k)$的增长性，证实了Mohar和Wu2018年的相关猜想。
A strong edge-coloring of a graph $G$ is an edge-coloring such that no two edges of distance at most two receive the same color. The strong chromatic index $χ'_s(G)$ is the minimum number of colors in a strong edge-coloring of $G$. P. Erdős and J. Nešetřil conjectured in 1985 that $χ'_s(G)$ is bounded above by $\frac54Δ^2$ when $Δ$ is even and $\frac14(5Δ^2-2Δ+1)$ when $Δ$ is odd, where $Δ$ is the maximum degree of $G$. In this paper, we give an algorithm that uses at most $2Δ^2-3Δ+2$ colors for graphs with girth at least $5$. And in particular, we prove that any graph with maximum degree $Δ=5…
A coloring of edges of a graph $G$ is injective if for any two distinct edges $e_1$ and $e_2$, the colors of $e_1$ and $e_2$ are distinct if they are at distance $1$ in $G$ or in a common triangle. Naturally, the injective chromatic index of $G$, $χ'_{inj}(G)$, is the minimum number of colors needed for an injective edge-coloring of $G$. We study how large can be the injective chromatic index of $G$ in terms of maximum degree of $G$ when we have restrictions on girth and/or chromatic number of $G$. We also compare our bounds with analogous bounds on the strong chromatic index.
We introduce the fractional version of oriented coloring and initiate its study. We prove some basic results and study the parameter for directed cycles and sparse planar graphs. In particular, we show that for every $ε> 0$, there exists an integer $g_ε \geq 12$ such that any oriented planar graph having girth at least $g_ε$ has fractional oriented chromatic number at most $4+ε$. Whereas, it is known that there exists an oriented planar graph having girth at least $g_ε$ with oriented chromatic number equal to $5$. We also study the fractional oriented chromatic number of directed cycles and pr…
Motivated by a recent conjecture of the first author, we prove that every properly coloured triangle-free graph of chromatic number $χ$ contains a rainbow independent set of size $\lceil\frac12χ\rceil$. This is sharp up to a factor $2$. This result and its short proof have implications for the related notion of chromatic discrepancy. Drawing inspiration from both structural and extremal graph theory, we conjecture that every triangle-free graph of chromatic number $χ$ contains an induced cycle of length $Ω(χ\logχ)$ as $χ\to\infty$. Even if one only demands an induced path of length $Ω(χ\logχ)$…
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