Given a graph G = (V,E) and a “cost function” $f:2^V\to ℝ$ (provided by an oracle), the problem [PCliqW] consists in finding a partition into cliques of V(G) of minimum cost. Here, the cost of a partition is the sum of the costs of the cliques in the partition. We provide a polynomial time dynamic program for the case where G is an interval graph and f belongs to a subclass of submodular set functions, which we call “value-polymatroidal”. This provides a common solution for various generalizations of the...

Let G = (V,E) be a simple undirected graph. A forest F ⊆ E of G is said to be clique-connecting if each tree of F spans a clique of G. This paper adresses the clique-connecting forest polytope. First we give a formulation and a polynomial time separation algorithm. Then we show that the nontrivial nondegenerate facets of the stable set polytope are facets of the clique-connecting polytope. Finally we introduce a family of rank inequalities which are facets, and which generalize the clique inequalities. ...

A graph G on a topological space X as its set of vertices is clopen if the edge relation of G is a clopen subset of X² without the diagonal. We study clopen graphs on Polish spaces in terms of their finite induced subgraphs and obtain information about their cochromatic numbers. In this context we investigate modular profinite graphs, a class of graphs obtained from finite graphs by taking inverse limits. This continues the investigation of continuous colorings on Polish spaces and their homogeneity...

Let G be a vertex colored graph. The minimum number χ(G) of colors needed for coloring of a graph G is called the chromatic number. Recently, Adiga et al. [1] have introduced the concept of color energy of a graph Ec(G) and computed the color energy of few families of graphs with χ(G) colors. In this paper we derive explicit formulas for the color energies of the unitary Cayley graph Xn, the complement of the colored unitary Cayley graph (Xn)c and some gcd-graphs.

Les modèles classiques de coloration doivent leur notoriété en grande partie à leurs applications à des problèmes de type emploi du temps ; nous présentons les concepts de base des colorations ainsi qu’une série de variations et de généralisations motivées par divers problèmes d’ordonnancement dont les élaborations d’horaires scolaires. Quelques algorithmes exacts et heuristiques seront présentés et nous esquisserons des méthodes basées sur la recherche Tabou pour trouver des solutions approchées...

The classical colouring models are well known thanks in large part to their applications to scheduling type problems; we describe the basic concepts of colourings together with a number of variations and generalisations arising from scheduling problems such as the creation of school schedules. Some exact and heuristic algorithms will be presented, and we will sketch solution methods based on tabu search to find approximate solutions to large problems. Finally we will also mention the use...

La généralisation des nombres chromatiques ${\chi }_n\left(G\right)$ de Stahl a été un premier thème de travail avec François et a abouti à l’introduction de la notion de colorations généralisées et leurs nombres chromatiques associés, notées ${\chi }_n^{p,q}\left(G\right)$. Cette nouvelle notion a permis d’une part, d’infirmer avec Payan une conjecture posée par Brigham et Dutton, et d’autre part, d’étendre de manière naturelle la formule de récurrence de Stahl aux nombres chromatiques ${\chi }_n^{0,q}\left(G\right)$. Cette relation s’exprime comme ${\chi }_n^{0,q}\left(G\right)\ge {\chi }_{n-1}^{0,q}\left(G\right)+2$. La conjecture de Bouchet...

A color-bounded hypergraph is a hypergraph (set system) with vertex set X and edge set = E₁,...,Eₘ, together with integers $s_i$ and $t_i$ satisfying $1\le s_i\le t_i\le \left|E_i\right|$ for each i = 1,...,m. A vertex coloring φ is proper if for every i, the number of colors occurring in edge $E_i$ satisfies $s_i\le \left|\phi \left(E_i\right)\right|\le t_i$. The hypergraph ℋ is colorable if it admits at least one proper coloring. We consider hypergraphs ℋ over a “host graph”, that means a graph G on the same vertex set X as ℋ, such that each $E_i$ induces a connected subgraph in G. In the current...

We show that the minimum chromatic number of a product of two $n$-chromatic graphs is either bounded by 9, or tends to infinity. The result is obtained by the study of coloring iterated adjoints of a digraph by iterated antichains of a poset.

If rooms in an office building are allowed to be any rectangular solid, how many colors does it take to paint any configuration of rooms so that no two rooms sharing a wall or ceiling/floor get the same color? In this work, we provide a new construction which shows this number can be arbitrarily large.

This note relates to bounds on the chromatic number χ(ℝn) of the Euclidean space, which is the minimum number of colors needed to color all the points in ℝn so that any two points at the distance 1 receive different colors. In [6] a sequence of graphs Gn in ℝn was introduced showing that . For many years, this bound has been remaining the best known bound for the chromatic numbers of some lowdimensional spaces. Here we prove that and find an exact formula for the chromatic number in the case of...