scholarly journals The Diachromatic Number of Digraphs

10.37236/7807 ◽  
2018 ◽  
Vol 25 (3) ◽  
Author(s):  
Gabriela Araujo-Pardo ◽  
Juan José Montellano-Ballesteros ◽  
Mika Olsen ◽  
Christian Rubio-Montiel
Keyword(s):  
Chromatic Number ◽  
Directed Graphs ◽  
Interpolation Property ◽  
Acyclic Coloring ◽  
Chromatic Class

We consider the extension to directed graphs of the concept of achromatic number in terms of acyclic vertex colorings. The achromatic number have been intensely studied since it was introduced by Harary, Hedetniemi and Prins in 1967. The dichromaticnumber is a generalization of the chromatic number for digraphs defined by Neumann-Lara in 1982. A coloring of a digraph is an acyclic coloring if each subdigraph induced by each chromatic class is acyclic, and a coloring is complete if for any pair of chromatic classes $x,y$, there is an arc from $x$ to $y$ and an arc from $y$ to $x$. The dichromatic and diachromatic numbers are, respectively, the smallest and the largest number of colors in a complete acyclic coloring. We give some general results for the diachromatic number and study it for tournaments. We also show that the interpolation property for complete acyclic colorings does hold and establish Nordhaus-Gaddum relations.

On the Basis and Chromatic Number of a Graph

1966 ◽  
Vol 18 ◽  
pp. 969-973
Author(s):  
C. J. Everett
Keyword(s):  
Chromatic Number ◽  
Directed Graphs ◽  
Cardinal Number ◽  
Ordered Sets ◽  
Basis Theorem ◽  
Arbitrary Collection

The basis theorem for directed graphs is, in effect, a result on weakly ordered sets, and, in §1, a proof is given, based on Zorn's lemma, that generalizes, and perhaps clarifies the exposition in (1, Chapter 2). In §2, a graph G* is defined, on an arbitrary collection Q of non-void subsets of a set X (which includes all its one-element subsets), in such a way that the partitions of X into Q-sets correspond to the kernels of G*. Applied to the collection Q of non-null internally stable subsets of a graph G without loops, this identifies the chromatic number of G with the least cardinal number of any kernel of G*.

scholarly journals Oriented Coloring on Recursively Defined Digraphs

Algorithms ◽  
2019 ◽  
Vol 12 (4) ◽  
pp. 87 ◽  
Author(s):  
Frank Gurski ◽  
Dominique Komander ◽  
Carolin Rehs
Keyword(s):  
Chromatic Number ◽  
Linear Time ◽  
Directed Graphs ◽  
Graph Isomorphism ◽  
Oriented Graph ◽  
Time Algorithm ◽  
Isomorphism Problem ◽  
Np Hard ◽  
Graph Isomorphism Problem ◽  
Acyclic Digraph

Coloring is one of the most famous problems in graph theory. The coloring problem on undirected graphs has been well studied, whereas there are very few results for coloring problems on directed graphs. An oriented k-coloring of an oriented graph G = ( V , A ) is a partition of the vertex set V into k independent sets such that all the arcs linking two of these subsets have the same direction. The oriented chromatic number of an oriented graph G is the smallest k such that G allows an oriented k-coloring. Deciding whether an acyclic digraph allows an oriented 4-coloring is NP-hard. It follows that finding the chromatic number of an oriented graph is an NP-hard problem, too. This motivates to consider the problem on oriented co-graphs. After giving several characterizations for this graph class, we show a linear time algorithm which computes an optimal oriented coloring for an oriented co-graph. We further prove how the oriented chromatic number can be computed for the disjoint union and order composition from the oriented chromatic number of the involved oriented co-graphs. It turns out that within oriented co-graphs the oriented chromatic number is equal to the length of a longest oriented path plus one. We also show that the graph isomorphism problem on oriented co-graphs can be solved in linear time.

scholarly journals On the Chromatic Thresholds of Hypergraphs

2015 ◽  
Vol 25 (2) ◽  
pp. 172-212
Author(s):  
JÓZSEF BALOGH ◽  
JANE BUTTERFIELD ◽  
PING HU ◽  
JOHN LENZ ◽  
DHRUV MUBAYI
Keyword(s):  
Chromatic Number ◽  
Minimum Degree ◽  
Directed Graphs ◽  
Open Problems ◽  
Fibre Bundles ◽  
Uniform Hypergraphs ◽  
The Family ◽  
Turán Number ◽  
Chromatic Thresholds ◽  
Extremal Set

Let $\mathcal{F}$ be a family of r-uniform hypergraphs. The chromatic threshold of $\mathcal{F}$ is the infimum of all non-negative reals c such that the subfamily of $\mathcal{F}$ comprising hypergraphs H with minimum degree at least $c \binom{| V(H) |}{r-1}$ has bounded chromatic number. This parameter has a long history for graphs (r = 2), and in this paper we begin its systematic study for hypergraphs.Łuczak and Thomassé recently proved that the chromatic threshold of the so-called near bipartite graphs is zero, and our main contribution is to generalize this result to r-uniform hypergraphs. For this class of hypergraphs, we also show that the exact Turán number is achieved uniquely by the complete (r + 1)-partite hypergraph with nearly equal part sizes. This is one of very few infinite families of non-degenerate hypergraphs whose Turán number is determined exactly. In an attempt to generalize Thomassen's result that the chromatic threshold of triangle-free graphs is 1/3, we prove bounds for the chromatic threshold of the family of 3-uniform hypergraphs not containing {abc, abd, cde}, the so-called generalized triangle.In order to prove upper bounds we introduce the concept of fibre bundles, which can be thought of as a hypergraph analogue of directed graphs. This leads to the notion of fibre bundle dimension, a structural property of fibre bundles that is based on the idea of Vapnik–Chervonenkis dimension in hypergraphs. Our lower bounds follow from explicit constructions, many of which use a hypergraph analogue of the Kneser graph. Using methods from extremal set theory, we prove that these Kneser hypergraphs have unbounded chromatic number. This generalizes a result of Szemerédi for graphs and might be of independent interest. Many open problems remain.

scholarly journals Harmonious chromatic number of directed graphs

2013 ◽  
Vol 161 (3) ◽  
pp. 369-376 ◽  
Author(s):  
Keith J. Edwards
Keyword(s):  
Chromatic Number ◽  
Directed Graphs

scholarly journals Acyclic Coloring of Graphs of Maximum Degree $\Delta$

2005 ◽  
Vol DMTCS Proceedings vol. AE,... (Proceedings) ◽  
Author(s):  
Guillaume Fertin ◽  
André Raspaud
Keyword(s):  
Chromatic Number ◽  
Upper Bound ◽  
Maximum Degree ◽  
Linear Time ◽  
Time Algorithm ◽  
Linear Time Algorithm ◽  
Acyclic Coloring ◽  
International Audience ◽  
Better Than

International audience An acyclic coloring of a graph $G$ is a coloring of its vertices such that: (i) no two neighbors in $G$ are assigned the same color and (ii) no bicolored cycle can exist in $G$. The acyclic chromatic number of $G$ is the least number of colors necessary to acyclically color $G$, and is denoted by $a(G)$. We show that any graph of maximum degree $\Delta$ has acyclic chromatic number at most $\frac{\Delta (\Delta -1) }{ 2}$ for any $\Delta \geq 5$, and we give an $O(n \Delta^2)$ algorithm to acyclically color any graph of maximum degree $\Delta$ with the above mentioned number of colors. This result is roughly two times better than the best general upper bound known so far, yielding $a(G) \leq \Delta (\Delta -1) +2$. By a deeper study of the case $\Delta =5$, we also show that any graph of maximum degree $5$ can be acyclically colored with at most $9$ colors, and give a linear time algorithm to achieve this bound.

scholarly journals An acyclic analogue to Heawood's theorem

1978 ◽  
Vol 19 (2) ◽  
pp. 163-166 ◽  
Author(s):  
Michael O. Albertson ◽  
David M. Berman
Keyword(s):  
Chromatic Number ◽  
Upper Bound ◽  
Acyclic Coloring ◽  
Minimum Value ◽  
Maximum Value

The concept of acyclic coloring was introduced by Grünbaum [5] and is a generalization of point arboricity.A proper k-coloring of the vertices of a graph Gis said to be acyclic if G contains no two-colored cycle. The acyclic chromatic number of a graph G, denoted by a(G), is the minimum value of k for which G has an acyclic k-coloring. Let a(n) denote the maximum value of the acyclic chromatic number among all graphs of genus n. In [5], Grünbaum conjectured that a(0) = 5 and proved that a(0)≤9. The conjecture was proved by Borodin [3] after the upper bound was improved three times in [7], [1] and [6]. In [2], we proved that a(1)≤a(0) + 3. The purpose of this paper is to prove the followingTheorem. Any graph of genus n>0 can be acyclically colored with 4n + 4 colors.It is not known for any n>0 whether a(n)>H(n), the Heawood number [8].

scholarly journals Critical Graphs for Acyclic Colorings

1978 ◽  
Vol 21 (1) ◽  
pp. 115-116
Author(s):  
David M. Berman
Keyword(s):  
Chromatic Number ◽  
Proper Coloring ◽  
Acyclic Coloring ◽  
Critical Graphs ◽  
Minimum Number ◽  
Proper Subgraph

The concept of acyclic colorings of graphs, introduced by Grunbaum [2], is a generalization of point-arboricity. An acyclic coloring of a graph is a proper coloring of its points such that there is no two-colored cycle. We denote by a(G), the acyclic chromatic number of a graph G, the minimum number of colors for an acyclic coloring of G. We call G k-critical if a(G) = fc but a(G′) for any proper subgraph G′. For all notation and terminology not defined here, see Harary [3].

Facets based on cycles and cliques for the acyclic coloring polytope

2020 ◽  
Vol 54 (6) ◽  
pp. 1863-1874
Author(s):  
Mónica Braga ◽  
Javier Marenco
Keyword(s):  
Integer Programming ◽  
Chromatic Number ◽  
Valid Inequalities ◽  
Programming Formulation ◽  
Acyclic Coloring ◽  
Coloring Problem ◽  
Integer Programming Formulation ◽  
Minimum Number ◽  
Np Complete

A coloring of a graph is an assignment of colors to its vertices such that any two vertices receive distinct colors whenever they are adjacent. An acyclic coloring is a coloring such that no cycle receives exactly two colors, and the acyclic chromatic number χA(G) of a graph G is the minimum number of colors in any such coloring of G. Given a graph G and an integer k, determining whether χA(G) ≤ k or not is NP-complete even for k = 3. The acyclic coloring problem arises in the context of efficient computations of sparse and symmetric Hessian matrices via substitution methods. In a previous work we presented facet-inducing families of valid inequalities based on induced even cycles for the polytope associated to an integer programming formulation of the acyclic coloring problem. In this work we continue with this study by introducing new families of facet-inducing inequalities based on combinations of even cycles and cliques.

scholarly journals Game Colouring Directed Graphs

10.37236/283 ◽  
2010 ◽  
Vol 17 (1) ◽  
Author(s):  
Daqing Yang ◽  
Xuding Zhu
Keyword(s):  
Planar Graph ◽  
Chromatic Number ◽  
Lower Bounds ◽  
Directed Graphs ◽  
Upper And Lower Bounds ◽  
Outerplanar Graph ◽  
Game Chromatic Number

In this paper, a colouring game and two versions of marking games (the weak and the strong) on digraphs are studied. We introduce the weak game chromatic number $\chi_{\rm wg}(D)$ and the weak game colouring number ${\rm wgcol}(D)$ of digraphs $D$. It is proved that if $D$ is an oriented planar graph, then $\chi_{\rm wg}(D)$ $\le {\rm wgcol}(D) \le 9$, and if $D$ is an oriented outerplanar graph, then $\chi_{\rm wg}(D)$ $\le {\rm wgcol}(D) \le 4$. Then we study the strong game colouring number ${\rm sgcol}\left( D \right)$ (which was first introduced by Andres as game colouring number) of digraphs $D$. It is proved that if $D$ is an oriented planar graph, then ${\rm sgcol}\left( D \right) \le 16$. The asymmetric versions of the colouring and marking games of digraphs are also studied. Upper and lower bounds of related parameters for various classes of digraphs are obtained.

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