Zeros of Chromatic and Flow Polynomials of Graphs
Bill Jackson
本文综述了图的色多项式、流多项式和拟阵的特征多项式的零分布的有关结果和定理。
We survey results and conjectures concerning the zero distribution of chromatic and flow polynomials of graphs, and characteristic polynomials of matroids.
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Regions without complex zeros for chromatic polynomials on graphs with bounded degree
本文证明了最大度为$\D$的有限图$\mathbb{G}$的色多项式$P_\mathbb{G}(q)$对于具有$$ C^*(\D) = \min_{0<x<2^{1\over \D}-1} {(1+x)^{\D-1}\over x [2-(1+x)^\D]} $$的$\card q\ge C^*(\D)$是无零点的，改进了Sokal（2001）和Borgs（2005）的结果.此外，我们提出了一个加强这个条件的图没有三角形自由顶点。
Chromatic Polynomials of Planar Triangulations, the Tutte Upper Bound, and Chromatic Zeros
Tutte证明了如果$G_{pt}$是一个平面三角剖分，$P(G_{pt},q)$是它的色多项式，则有$|P(G_{pt},τ+1)| \le (τ-1)^{n-5}$，其中$τ=(1+\sqrt{5} \,)/2$和$n$是$G_{pt}$中的顶点数.在这里，我们研究的比率$r(G_{pt})=|P(G_{pt},τ+1)|/(τ-1)^{n-5}$的各种平面三角剖分。我们构建无限递归家庭的平面三角形$G_{pt,m}$依赖于一个参数$m$线性相关的$n$，并表明，如果$P(G_{pt,m},q)$只涉及一个单一的权力的多项式，然后$r(G_{pt,m})$接近零指数快速$n \to \infty$。我们还构建了无限递归家庭，其中$P(G_{pt,m},q)$是某些功能的权力和显示，对于这些，$r(G_{pt,m})$可能会接近一个有限的非零常数为$n \to \infty$。研究了Tutte上界与观测到的$τ+1$附近的色零点之间的联系.我们报告了第一个已知的图，其中最接近$τ+1$的零（S）不是真实的，而是一个复共轭对。最后，我们讨论了与非零基态熵的Potts反铁磁体在这些家庭的图。
Chromatic polynomials and related graph invariants are central objects in both graph theory and statistical physics. Computational difficulties, however, have so far restricted studies of such polynomials to graphs that were either very small, very sparse or highly structured. Recent algorithmic advances (Timme et al 2009 New J. Phys. 11 023001) now make it possible to compute chromatic polynomials for moderately sized graphs of arbitrary structure and number of edges. Here we present chromatic polynomials of ensembles of random graphs with up to 30 vertices, over the entire range of edge dens…
Chromatic polynomials of simplicial complexes
我们考虑单纯复形的s-色多项式，图的色多项式的高维类似物.
This paper describes how many known graph polynomials arise from the coefficients of chromatic symmetric function expansions in different bases, and studies a new polynomial arising by expanding over a basis given by chromatic symmetric functions of trees.
Using the definition of colouring of $2$-edge-coloured graphs derived from $2$-edge-coloured graph homomorphism, we extend the definition of chromatic polynomial to $2$-edge-coloured graphs. We find closed forms for the first three coefficients of this polynomial that generalize the known results for the chromatic polynomial of a graph. We classify those $2$-edge-coloured graphs that have a chromatic polynomial equal to the chromatic polynomial of the underlying graph, when every vertex is incident to edges of both colours. Finally, we examine the behaviour of the roots of this polynomial, hig…
Chromatic Polynomials, Potts Models and All That
q-状态Potts模型可以定义在任意有限图上，其配分函数包含了图的色多项式、流多项式和可靠性多项式等重要信息.波茨配分函数的复零点对统计力学家和组合学家都有意义。我对所有这些问题作了教学性的介绍，然后简述了两个最近的结果：（a）构造了一个可数的平面图族，其色零点在整个复q平面上都是稠密的，除了圆盘|Q-1| < 1。(b)证明了色多项式（或反铁磁Potts模型配分函数）的q平面零点在图的最大度方面的普遍上界。
In this paper, we present some properties on chromatic polynomials of hypergraphs which do not hold for chromatic polynomials of graphs. We first show that chromatic polynomials of hypergraphs have all integers as their zeros and contain dense real zeros in the set of real numbers. We then prove that for any multigraph $G=(V,E)$, the number of totally cyclic orientations of $G$ is equal to the value of $|P(H,-1)|$, where $P(H,λ)$ is the chromatic polynomial of a hypergraph $H$ which is constructed from $G$. Finally we show that the multiplicity of root "$0$" of $P(H,λ)$ may be at least $2$ for…
Chromatic polynomials of signed graphs and dominating-vertex deletion formulae
本文通过构造共享底层图和色多项式的非切换同构符号图回答了 Zaslavsky 的问题，推导了符号完全图连接的色多项式闭式表达，并引入双变量色多项式及其支配顶点删除递归公式，证明其在特定符号阈值图族中可刻画同构。
In this paper, we prove that $(-\infty, 0)$ is a zero-free interval for chromatic polynomials of a family ${\cal L}_0$ of hypergraphs and $(0, 1)$ is a zero-free interval for chromatic polynomials of a subfamily ${\cal L}_0'$ of ${\cal L}_0$ of hypergraphs. These results extend known results on zero-free intervals of chromatic polynomials of graphs and hypergraphs.
We use a polynomial decomposition result by Stapledon to show that the numerator polynomial of the Ehrhart series of an open polytope is the difference of two symmetric polynomials with nonnegative integer coefficients. We obtain a related decomposition for order polytopes and for the numerator polynomial of the corresponding series for chromatic polynomials. The nonnegativity of the coefficients in such decompositions provide inequalities satisfied by the coefficients of chromatic polynomials for any simple graph.
Some applications of the chromatic polynomials
色多项式在不同的框架中有重要的应用，特别是在图论和计数组合数学中。本文的目的是建立图的色多项式的系数的一些性质。给出了第二类限制Stirling数的三个应用。
We express the integral form Macdonald polynomials as a weighted sum of Shareshian and Wachs' chromatic quasisymmetric functions of certain graphs. Then we use known expansions of these chromatic quasisymmetric functions into Schur and power sum symmetric functions to provide Schur and power sum formulas for the integral form Macdonald polynomials. Since the (integral form) Jack polynomials are a specialization of integral form Macdonald polynomials, we obtain analogous formulas for Jack polynomials as corollaries.
On the degree-chromatic polynomial of a tree
图$G$的度色多项式$Pm(G,k)$计算其中没有顶点具有相同颜色的$m$邻顶点的$k$ -着色的数目。证明了Humpert和Martin关于树的次数色多项式的首项的猜想。
Set maps, umbral calculus, and the chromatic polynomial
色多项式的一些重要性质也适用于满足p_S（x+y）= \sum_{T\uplus U=S} p_T（x）p_U（y）的多项式集映射.利用投影演算，我们给出了这样一个集合映射关于任意二项式型多项式序列的展开公式。这导致了色多项式的一些新的展开式。我们还描述了阿贝尔多项式的集合映射推广。
Motivated by the study of Macdonald polynomials, J. Haglund and A. Wilson introduced a nonsymmetric polynomial analogue of the chromatic quasisymmetric function called the \emph{chromatic nonsymmetric polynomial} of a Dyck graph. We give a positive expansion for this polynomial in the basis of fundamental slide polynomials using recent work of Assaf-Bergeron on flagged $(P,ρ)$-partitions. We then derive the known expansion for the chromatic quasisymmetric function of Dyck graphs in terms of Gessel's fundamental basis by taking a backstable limit of our expansion.
On Multivariate Chromatic Polynomials of Hypergraphs and Hyperedge Elimination
本文考虑超图的多元超边消去多项式和多元色多项式。第一组多项式是定义在删除收缩提取递归，以前研究的图由Averbouch，Godlin和Makowsky。多元色多项式是一个等价的多项式定义的着色，并推广了上边界多项式的Crapo，和二元色多项式的Dohmen，Pönitz和Tittman。我们表明，这些新的多项式恢复多项式列举超边缘覆盖，匹配，横截，和部分超图的专业化。我们还证明了多项式可以定义在超图的键格上的莫比乌斯反演，以及计算这些多项式的各类超图。
Let G be a graph, and let $χ$G be its chromatic polynomial. For any non-negative integers i, j, we give an interpretation for the evaluation $χ$ (i) G (--j) in terms of acyclic orientations. This recovers the classical interpretations due to Stanley and to Green and Zaslavsky respectively in the cases i = 0 and j = 0. We also give symmetric function refinements of our interpretations, and some extensions. The proofs use heap theory in the spirit of a 1999 paper of Gessel.
Chromatic polynomials have been studied extensively, giving us results such as the Fundamental Reduction Theorem and closed formulas for the chromatic polynomials of common classes of graphs. Though, none of those extend to the context of fractional colorings. We thus present closed formulas for the "fractional" chromatic polynomial - a function that counts the number of distinct $b$-fold $λ$-colorings on a given graph - of complete graphs, trees and forests, as well as a generalized form of the Fundamental Reduction Theorem.
Let $G = (V,E)$ be a finite, simple, connected graph with chromatic polynomial $P_G(q)$. Sokal \cite{sokal} proved that the roots of the chromatic polynomial of $G$ are bounded in absolute value by $KD$ where, $D$ is the maximum degree of the graph and $7< K < 8$ is a constant. In this paper we generalize this result to uniform hypergraphs. To prove our results we will use the theory of the bounded exponential type graph polynomials.
Stanley's symmetrized chromatic polynomial is a generalization of the ordinary chromatic polynomial to a graph invariant with values in a ring of polynomials in infnitely many variables. The ordinary chromatic polynomial is a specialization of the Stanley's one. Our goal is to extend Stanley's chromatic polynomial to embedded graphs. In contrast to well-known extensions of, say, the Tutte polynomial from abstract to embedded graphs [5], we do not treat an embedded graph as an abstract graph endowed with additional information about the embedding. Instead, we consider the binary delta-matroid a…
The oriented chromatic polynomial of a oriented graph outputs the number of oriented $k$-colourings for any input $k$. We fully classify those oriented graphs for which the oriented graph has the same chromatic polynomial as the underlying simple graph, closing an open problem posed by Sopena. We find that such oriented graphs can be both identified and constructed in polynomial time as they are exactly the family of quasi-transitive oriented co-interval graphs. We study the analytic properties of this polynomial and show that there exist oriented graphs which have chromatic polynomials have r…
Inductive approach to chromatic and characteristic polynomials
本文提出一种新的归纳方法，用于计算简单图的色多项式和中心超平面排列的特征多项式。
Chromatic polynomials of complements of bipartite graphs
二部图是二部图的补图;因此，每个图都由两个由多条边连接的团组成。在本文中，我们研究这些图的色多项式的代数方面。本文导出了任意二分图的色多项式的一个公式，并利用这个公式给出了两个图具有相同分裂域的色多项式的条件。最后，我们使用一个bicliques的子类证明了三次情况下的$α+n$猜想，通过显示，任何三次整数$α$，有一个自然数$n$，使$α+n$是一个色根。
We study "holomorphic quadratic differentials" on graphs. We relate them to the reactive power in an LC circuit, and also to the chromatic polynomial of a graph. Specifically, we show that the chromatic polynomial $χ$ of a graph $G$, at negative integer values, can be evaluated as the degree of a certain rational mapping, arising from the defining equations for a holomorphic quadratic differential. This allows us to give an explicit integral expression for $χ(-k)$.
Given a group $G$ of automorphisms of a graph $Γ$, the orbital chromatic polynomial $OP_{Γ,G}(x)$ is the polynomial whose value at a positive integer $k$ is the number of orbits of $G$ on proper $k$-colorings of $Γ.$ In \cite{Cameron}, Cameron et. al. explore the roots of orbital chromatic polynomials, and in particular prove that orbital chromatic roots are dense in $\mathbb{R}$, extending Thomassen's famous result (see \cite{Thomassen}) that chromatic roots are dense in $[\frac{32}{27},\infty)$. Cameron et al \cite{Cameron} further conjectured that the real roots of the orbital chromatic pol…
It is proved that if $G$ is a graph containing a spanning tree with at most three leaves, then the chromatic polynomial of $G$ has no roots in the interval $(1,t_1]$, where $t_1 \approx 1.2904$ is the smallest real root of the polynomial $(t-2)^6 +4(t-1)^2(t-2)^3 -(t-1)^4$. We also construct a family of graphs containing such spanning trees with chromatic roots converging to $t_1$ from above. We employ the Whitney $2$-switch operation to manage the analysis of an infinite class of chromatic polynomials.
On Brenti's conjecture about the log-concavity of the chromatic polynomial
色多项式是图论中的一个重要研究对象。关于色多项式及与之相关的其它多项式的对数正态性，已有许多结果和说明，这些多项式的根的位置也得到了很好的研究。A. Sokal和C. Borgs提供了一个关于图的最高度的色多项式的根的绝对值的界。我们利用这个结果证明了一个由F.布伦蒂Brenti最初的猜想是色多项式在自然数上是对数凹的。保罗·西摩（Paul Seymour）提出了一个相反的例子来证明这一点。我们证明了图$G$的色多项式$P_G(q)$对所有的$q > CΔ+ 1$都是对数凹的，其中$Δ$表示$G$的最高度。我们还提供了一个例子，表明对于小于1的常数$C$，结果不成立。
Zaslavsky proved in 2012 that, up to switching isomorphism, there are six different signed Petersen graphs and that they could be told apart by their chromatic polynomials, by showing that the latter give distinct results when evaluated at 3. He conjectured that the six different signed Petersen graphs also have distinct zero-free chromatic polynomials, and that both types of chromatic polynomials have distinct evaluations at \emph{any} positive integer. We developed and executed a computer program (running in {\tt SAGE}) that efficiently determines the number of proper $k$-colorings for a giv…
We define the acyclic orientation polynomial of a graph to be the generating function for the sinks of its acyclic orientations. Stanley proved that the number of acyclic orientations is equal to the chromatic polynomial evaluated at $-1$ up to sign. Motivated by this link between acyclic orientations and the chromatic polynomial, we develop "acyclic orientation" analogues of theorems concerning the chromatic polynomial of Birkhoff, Whitney, and Greene-Zaslavsky. As an application, we provide a new proof for Stanley's sink theorem for chromatic symmetric functions $X_G$. This theorem gives a r…
A categorification for the signed chromatic polynomial
通过符号着色构造符号图的符号色多项式，并构建其分次上同调群，使得分次欧拉示性数等于符号色多项式，且上同调群满足对应于符号删除-收缩规则的长正合序列。
For $m \geq 3$ and $n \geq 1$, the $m$-cycle book graph $B(m,n)$ consists of $n$ copies of the cycle $C_m$ with one common edge. In this paper, we prove that (a) the number of switching non-isomorphic signed $B(m,n)$ is $n+1$, and (b) the chromatic number of a signed $B(m,n)$ is either 2 or 3. We also obtain explicit formulas for the chromatic polynomials and the zero-free chromatic polynomials of switching non-isomorphic signed book graphs.
A generalized vertex join of a graph is obtained by joining an arbitrary multiset of its vertices to a new vertex. We present a low-order polynomial time algorithm for finding the chromatic polynomials of generalized vertex joins of trees, and by duality we find the flow polynomials of arbitrary outerplanar graphs. We also present closed formulas for the chromatic and flow polynomials of vertex joins of cliques and cycles, otherwise known as "generalized wheel" graphs.
In 2015, Matthias Beck and his team developed a computer program in SAGE which efficiently determines the number of signed proper $k$-colorings for a given signed graph. In this article, we determine the number of different signatures on Book graph up to switching isomorphisms. We also find a recursive formula of the signed chromatic polynomials of signed Book graphs.
In recent years we have worked on a project involving poset topology, various analogues of Eulerian polynomials, and a refinement of Richard Stanley's chromatic symmetric function. Here we discuss how Stanley's ideas and results have influenced and inspired our own work.
We give a new interpretation of the chromatic polynomial of a simple graph G in terms of the Kac-Moody Lie algebra with Dynkin diagram G. We show that the chromatic polynomial is essentially the q-Kostant partition function of this Lie algebra evaluated on the sum of the simple roots. Applying the Peterson recurrence formula for root multiplicities, we obtain a new realization of the chromatic polynomial as a weighted sum of paths in the bond lattice of G.
We introduce the characteristic numbers and the chromatic polynomial of a tensor. Our approach generalizes and unifies the chromatic polynomial of a graph and of a matroid, characteristic numbers of quadrics in Schubert calculus, Betti numbers of complements of hyperplane arrangements and Euler characteristic of complements of determinantal hypersurfaces and the maximum likelihood degree for general linear concentration models in algebraic statistics.
We use a Dyck path model for unit-interval graphs to study the chromatic quasisymmetric functions introduced by Shareshian and Wachs, as well as vertical strip --- in particular, unicellular LLT polynomials. We show that there are parallel phenomena regarding $e$-positivity of these two families of polynomials. In particular, we give several examples where the LLT polynomials behave like a "mirror image" of the chromatic quasisymmetric counterpart. The Dyck path model is also extended to circular arc digraphs to obtain larger families of polynomials. This circular extensions of LLT polynomials…
The chromatic polynomial of fatgraphs and its categorification
基于Khovanov同调理论以及Jones多项式与图多项式之间的关系，我们构造了嵌入图的同调论，并由此得到了嵌入图的色多项式作为欧拉示性数.对于平面图，我们证明了我们的色同调可以从一个关联链接的Khovanov同调中恢复。我们应用这种连接与Khovanov同调，我们的色同调的无挠部分是独立的选择的平面嵌入的图。我们扩展我们的建设和分类的Bollobas Riordan多项式（Tutte多项式的概括嵌入图）。我们证明，我们的色同源性和Khovanov同源性的相关链接可以从这个分类。
An acyclic coloring of a digraph as defined by Neumann-Lara is a vertex-coloring such that no monochromatic directed cycles occur. Counting the number of such colorings with $k$ colors can be done by counting so-called Neumann-Lara-coflows (NL-coflows), which build a polynomial in $k$. We will present a representation of this polynomial using totally cyclic subdigraphs, which form a graded poset $Q$. Furthermore we will decompose our NL-coflow polynomial, which becomes the chromatic polynomial of a digraph by multiplication with the number of colors to the number of components, examining the s…
A Survey of Graph Polynomials Related to the Chromatic Polynomial
本文综述了与色多项式相关的图多项式，包括其与Tutte多项式、特征多项式及流多项式的联系，并回顾了相关主要研究成果。
The purpose of this paper is twofold. Firstly, we generalize the notion of characteristic polynomials of hyperplane and toric arrangements to those of certain abelian Lie group arrangements. Secondly, we give two interpretations for the chromatic quasi-polynomials and their constituents through subspace and toric viewpoints.
Closed-form expansions for the bivariate chromatic polynomial of paths and cycles
我们建立了封闭形式的展开式的数量染色的道路或循环的n个顶点的颜色从1，...，x，使得邻顶点被不同地着色或具有来自y+1，. X.
Link invariants, the chromatic polynomial and the Potts model
我们研究链接不变量之间的联系，色多项式，几何表示模型的统计力学，和他们共同的基本代数结构。我们建立了几个代数之间的关系和它们相关的组合和拓扑量。特别地，我们定义了色代数，它的Markov迹是一个关联图的色多项式χ_Q，并且我们给出了这种新的代数方法在色多项式的组合性质上的应用.在统计力学中，这个代数出现在Q态Potts模型的低温膨胀中。建立了色代数与SO（3）Birman-Murakami-Wenzl代数之间的关系，它是SO（3）Kauffman多项式与色多项式之间的对应关系的代数级模拟.
A bivariate chromatic polynomial for signed graphs
本文研究了Dohmen-Pönitz-Tittmann的二元色多项式$c_Γ(k,l)$，它对图$Γ$的所有$(k+l)$ -着色进行计数，使得如果邻顶点是$\le k$，则它们得到不同的颜色.我们的第一个贡献是将$c_Γ(k,l)$推广到有符号图上，得到了一个包含-排除公式和几个特殊的评价，例如，到编码平衡子图的多项式。我们的第二个目标是推导出组合互易定理$c_Γ(k,l)$和它的签名图类似物，让人想起斯坦利的互易定理连接色多项式的非循环方向。
We find families of simplicial complexes where the simplicial chromatic polynomials defined by Cooper--de Silva--Sazdanovic \cite{CdSS} are Hilbert series of Stanley--Reisner rings of auxiliary simplicial complexes. As a result, such generalized chromatic polynomials are determined by $h$-vectors of auxiliary simplicial complexes. In addition to generalizing related results on graphs and matroids, the simplicial complexes used allow us to consider problems that are not necessarily analogues of those considered for graphs. Some examples include supports of cyclotomic polynomials, log concavity …
$λ$-Chromatic Polynomials and Polytope Geometry
本文研究图的$λ$-色多项式，该多项式计数使用给定有限颜色集的不同$L(2,1)$-着色数量，通过内部-外部多面体框架下的格点枚举给出组合解释，并利用格路枚举和块-间隙技术计算完全图、完全二部图及多部图的$λ$-色多项式。
Stanley in his paper [Stanley, Richard P.: Acyclic orientations of graphs In: Discrete Mathematics 5 (1973), Nr. 2, S. 171-178.] provided interpretations of the chromatic polynomial when it is substituted with negative integers. Greene and Zaslavsky interpreted the coefficients of the chromatic polynomial in [Greene, Curtis ; Zaslavsky, Thomas: On the interpretation of Whitney numbers through arrangements of hyperplanes, zonotopes, non-Radon partitions, and orientations of graphs. In: Transactions of the American Mathematical Society 280 (1983), jan, Nr. 1, S. 97-97.]. We shall develop an invo…
Transfer Matrices and Partition-Function Zeros for Antiferromagnetic Potts Models III. Triangular-Lattice Chromatic Polynomial
本文研究了m个\times n个三角网格条的色多项式P_G（q），其宽度m <= 12_P，9_F（分别具有周期性或自由横向边界条件）和任意长度n（具有自由纵向边界条件）。色多项式给出了q态Potts反铁磁体的配分函数的零温度极限。我们计算的转移矩阵的Fortuin-Kasteleyn表示，并获得相应的累积集的色零点在复q平面上的极限n \to \infty。我们重新计算的极限曲线巴克斯特在热力学极限m，n \to \infty，并找到新的有趣的功能与可能的物理后果。最后，我们分析了孤立极限点及其与Beraha数的关系。
A new polynomial on compositions of integers: on distinguishing caterpillars from their symmetric chromatic function
本文提出了一种判定两棵非同构毛毛虫树是否具有相同对称函数的代数方法--色多项式的推广。在整数上的所有组合的集合上，我们引入：一个操作，我们称之为组合乘积;和一个组合多项式，我们称之为组合格多项式或L-多项式，它模仿了Noble和Welsh的加权图多项式。我们证明了一个唯一的不可约分解定理，并建立了一个连接的L-多项式的组合和它的不可约分解，即扭转不可约因子不改变L，并猜想这是唯一的方式产生这样的组合。最后，我们找到一个充分条件，两个毛毛虫有一个不同的对称函数推广的色多项式，并使用这个条件来表明，如果我们的猜想是成立的，那么对称函数推广的色多项式区分一大类毛毛虫。
Chromatic Polynomial, Colered Jones Function and q-Binomial Counting
在一般弦图类中定义了图的q-色函数，给出了它的一些性质，并给出了一些公式。然后，我们把q-色函数与纽结的色琼斯函数联系起来。这导致了一个奇怪的表达的有色琼斯函数的结图作为一个“缺陷色运营商”适用于幂级数的系数是线性组合的弦图构建从“流”的减少结图。
The main result of this paper is the introduction of marked graphs and the marked graph polynomials ($M$-polynomial) associated with them. These polynomials can be defined via a deletion-contraction operation. These polynomials are a generalization of the $W$-polynomial introduced by Noble and Welsh and a specialization of the $\mathbf{V}$-polynomial introduced by Ellis-Monaghan and Moffatt. In addition, we describe an important specialization of the $M$-polynomial which we call the $D$-polynomial. Furthermore, we give an efficient algorithm for computing the chromatic symmetric function of a …
A graph G is H-free if it has no induced subgraph isomorphic to H. We prove that a $P_5$-free graph with clique number $ω\ge 3$ has chromatic number at most $ω^{\log_2(ω)}$. The best previous result was an exponential upper bound $(5/27)3^ω$, due to Esperet, Lemoine, Maffray, and Morel. A polynomial bound would imply that the celebrated Erdos-Hajnal conjecture holds for $P_5$, which is the smallest open case. Thus there is great interest in whether there is a polynomial bound for $P_5$-free graphs, and our result is an attempt to approach that.
A famous and wide-open problem, going back to at least the early 1970's, concerns the classification of chromatic polynomials of graphs. Toward this classification problem, one may ask for necessary inequalities among the coefficients of a chromatic polynomial, and we contribute such inequalities when a chromatic polynomial $χ_G(n) = χ^*_0 \binom {n+d} d + χ^*_1 \binom {n+d-1} d + \dots + χ^*_d \binom n d$ is written in terms of a binomial-coefficient basis. For example, we show that $χ^*_{ j } \le χ^*_{ d-j }$, for $0 \le j \le \frac{ d }{ 2 }$. Similar results hold for flow and tension polyn…
In this work, we generalize and utilize the linear relations of LLT polynomials introduced by Lee \cite{Lee}. By using the fact that the chromatic quasisymmetric functions and the unicellular LLT polynomials are related via plethystic substitution and thus they satisfy the same linear relations, we can apply the linear relations to both sets of functions. As a result, in the chromatic quasisymmetric function side, we find a class of $e$-positive graphs, called \emph{melting lollipop graphs}, and explicitly prove the $e$-unimodality. In the unicellular LLT side, we obtain Schur expansion formul…
A double star is a tree with two internal vertices. It is known that the Gyárfás-Sumner conjecture holds for double stars, that is, for every double star $H$, there is a function $f$ such that if $G$ does not contain $H$ as an induced subgraph then $χ(G)\le f(ω(G))$ (where $χ, ω$ are the chromatic number and the clique number of $G$). Here we prove that $f$ can be chosen to be a polynomial.
We present a new correspondence between acyclic orientations and coloring of a signed graph (symmetric graph). Goodall et al. introduced a bivariate chromatic polynomial $χ_G(k,l)$ that counts the number of signed colorings using colors $0,\pm1,\dots,\pm k$ along with $l-1$ symmetric colors $0_1,\dots,0_{l-1}$. We show that the evaluation of the bivariate chromatic polynomial $|χ_G(-1,2)|$ is equal to the number of acyclic orientations of the signed graph modulo the equivalence relation generated by swapping sources and sinks. We present three proofs of this fact, a proof using toric hyperplan…
A star edge coloring of a graph $G$ is a proper edge coloring of $G$ such that every path and cycle of length four in $G$ uses at least three different colors. The star chromatic index of a graph $G$, is the smallest integer $k$ for which $G$ admits a star edge coloring with $k$ colors. In this paper, we present a polynomial time algorithm that finds an optimum star edge coloring for every tree. We also provide some tight bounds on the star chromatic index of trees with diameter at most four, and using these bounds we find a formula for the star chromatic index of certain families of trees.
If $G$ is a $k$-chromatic graph of order $n$ then it is known that the chromatic polynomial of $G$, $π(G,x)$, is at most $x(x-1)\cdots (x-(k-1))x^{n-k} = (x)_{\downarrow k}x^{n-k}$ for every $x\in \mathbb{N}$. We improve here this bound by showing that \[ π(G,x) \leq (x)_{\downarrow k} (x-1)^{Δ(G)-k+1} x^{n-1-Δ(G)}\] for every $x\in \mathbb{N},$ where $Δ(G)$ is the maximum degree of $G$. Secondly, we show that if $G$ is a connected $k$-chromatic graph of order $n$ where $k\geq 4$ then $π(G,x)$ is at most $(x)_{\downarrow k}(x-1)^{n-k}$ for every real $x\geq n-2+\left( {n \choose 2} -{k \choose…
Let $G=(V,E)$ be a simple graph with $V=\{1,2,\cdots,n\}$ and $χ(G,x)$ be its chromatic polynomial. For an ordering $π=(v_1,v_2,\cdots,v_n)$ of elements of $V$, let $δ_G(π)$ be the number of $i$'s, where $1\le i\le n-1$, with either $v_i<v_{i+1}$ or $v_iv_{i+1}\in E$. Let ${\cal W}(G)$ be the set of subsets $\{a,b,c\}$ of $V$, where $a<b<c$, which induces a subgraph with $ac$ as its only edge. We show that ${\cal W}(G)=\emptyset$ if and only if $(-1)^nχ(G,-x)=\sum_π {x+δ_G(π)\choose n}$, where the sum runs over all $n!$ orderings $π$ of $V$. To prove this result, we establish an analogous resu…
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