Page 1 Next

Displaying 1 – 20 of 22

Showing per page

A class of weakly perfect graphs

H. R. Maimani, M. R. Pournaki, S. Yassemi (2010)

Czechoslovak Mathematical Journal

A graph is called weakly perfect if its chromatic number equals its clique number. In this note a new class of weakly perfect graphs is presented and an explicit formula for the chromatic number of such graphs is given.

Backbone colorings along stars and matchings in split graphs: their span is close to the chromatic number

Hajo Broersma, Bert Marchal, Daniel Paulusma, A.N.M. Salman (2009)

Discussiones Mathematicae Graph Theory

We continue the study on backbone colorings, a variation on classical vertex colorings that was introduced at WG2003. Given a graph G = (V,E) and a spanning subgraph H of G (the backbone of G), a λ-backbone coloring for G and H is a proper vertex coloring V→ {1,2,...} of G in which the colors assigned to adjacent vertices in H differ by at least λ. The algorithmic and combinatorial properties of backbone colorings have been studied for various types of backbones in a number of papers. The main outcome...

Choice-Perfect Graphs

Zsolt Tuza (2013)

Discussiones Mathematicae Graph Theory

Given a graph G = (V,E) and a set Lv of admissible colors for each vertex v ∈ V (termed the list at v), a list coloring of G is a (proper) vertex coloring ϕ : V → S v2V Lv such that ϕ(v) ∈ Lv for all v ∈ V and ϕ(u) 6= ϕ(v) for all uv ∈ E. If such a ϕ exists, G is said to be list colorable. The choice number of G is the smallest natural number k for which G is list colorable whenever each list contains at least k colors. In this note we initiate the study of graphs in which the choice number equals...

Comparing imperfection ratio and imperfection index for graph classes

Arie M. C. A. Koster, Annegret K. Wagler (2008)

RAIRO - Operations Research - Recherche Opérationnelle

Perfect graphs constitute a well-studied graph class with a rich structure, reflected by many characterizations with respect to different concepts. Perfect graphs are, for instance, precisely those graphs G where the stable set polytope STAB ( G ) coincides with the fractional stable set polytope QSTAB ( G ) . For all imperfect graphs G it holds that STAB ( G ) QSTAB ( G ) . It is, therefore, natural to use the difference between the two polytopes in order to decide how far an imperfect graph is away from being perfect. We discuss three...

Comparing Imperfection Ratio and Imperfection Index for Graph Classes

Arie M.C.A. Koster, Annegret K. Wagler (2009)

RAIRO - Operations Research

Perfect graphs constitute a well-studied graph class with a rich structure, reflected by many characterizations with respect to different concepts. Perfect graphs are, for instance, precisely those graphs G where the stable set polytope STAB(G) coincides with the fractional stable set polytope QSTAB(G). For all imperfect graphs G it holds that STAB(G) ⊂ QSTAB(G). It is, therefore, natural to use the difference between the two polytopes in order to decide how far an imperfect graph is away...

Conditions for β-perfectness

Judith Keijsper, Meike Tewes (2002)

Discussiones Mathematicae Graph Theory

A β-perfect graph is a simple graph G such that χ(G') = β(G') for every induced subgraph G' of G, where χ(G') is the chromatic number of G', and β(G') is defined as the maximum over all induced subgraphs H of G' of the minimum vertex degree in H plus 1 (i.e., δ(H)+1). The vertices of a β-perfect graph G can be coloured with χ(G) colours in polynomial time (greedily). The main purpose of this paper is to give necessary and sufficient conditions, in terms of forbidden induced subgraphs,...

Distance perfectness of graphs

Andrzej Włoch (1999)

Discussiones Mathematicae Graph Theory

In this paper, we propose a generalization of well known kinds of perfectness of graphs in terms of distances between vertices. We introduce generalizations of α-perfect, χ-perfect, strongly perfect graphs and we establish the relations between them. Moreover, we give sufficient conditions for graphs to be perfect in generalized sense. Other generalizations of perfectness are given in papers [3] and [7].

Le théorème fort des graphes parfaits

Gérard Cornuéjols (2005/2006)

Séminaire Bourbaki

Au début des années 60, Claude Berge a proposé deux conjectures sur les graphes parfaits. La première a été démontrée par Laci Lovász en 1972. La deuxième, dite conjecture forte des graphes parfaits, a fait couler beaucoup d’encre dans les 30 années qui ont suivi. Ce n’est qu’en 2002 qu’elle a été démontrée dans un article très impressionnant de 179 pages par Maria Chudnovsky, Neil Robertson, Paul Seymour et Robin Thomas. L’exposé présentera cette conjecture célèbre et donnera une idée de sa démonstration....

Minimal trees and monophonic convexity

Jose Cáceres, Ortrud R. Oellermann, M. L. Puertas (2012)

Discussiones Mathematicae Graph Theory

Let V be a finite set and 𝓜 a collection of subsets of V. Then 𝓜 is an alignment of V if and only if 𝓜 is closed under taking intersections and contains both V and the empty set. If 𝓜 is an alignment of V, then the elements of 𝓜 are called convex sets and the pair (V,𝓜 ) is called an alignment or a convexity. If S ⊆ V, then the convex hull of S is the smallest convex set that contains S. Suppose X ∈ ℳ. Then x ∈ X is an extreme point for X if X∖{x} ∈ ℳ. A convex geometry on a finite set is...

On a perfect problem

Igor E. Zverovich (2006)

Discussiones Mathematicae Graph Theory

We solve Open Problem (xvi) from Perfect Problems of Chvátal [1] available at ftp://dimacs.rutgers.edu/pub/perfect/problems.tex: Is there a class C of perfect graphs such that (a) C does not include all perfect graphs and (b) every perfect graph contains a vertex whose neighbors induce a subgraph that belongs to C? A class P is called locally reducible if there exists a proper subclass C of P such that every graph in P contains a local subgraph belonging...

Set colorings in perfect graphs

Ralucca Gera, Futaba Okamoto, Craig Rasmussen, Ping Zhang (2011)

Mathematica Bohemica

For a nontrivial connected graph G , let c : V ( G ) be a vertex coloring of G where adjacent vertices may be colored the same. For a vertex v V ( G ) , the neighborhood color set NC ( v ) is the set of colors of the neighbors of v . The coloring c is called a set coloring if NC ( u ) NC ( v ) for every pair u , v of adjacent vertices of G . The minimum number of colors required of such a coloring is called the set chromatic number χ s ( G ) . We show that the decision variant of determining χ s ( G ) is NP-complete in the general case, and show that χ s ( G ) can be...

Some Variations of Perfect Graphs

Magda Dettlaff, Magdalena Lemańska, Gabriel Semanišin, Rita Zuazua (2016)

Discussiones Mathematicae Graph Theory

We consider (ψk−γk−1)-perfect graphs, i.e., graphs G for which ψk(H) = γk−1(H) for any induced subgraph H of G, where ψk and γk−1 are the k-path vertex cover number and the distance (k − 1)-domination number, respectively. We study (ψk−γk−1)-perfect paths, cycles and complete graphs for k ≥ 2. Moreover, we provide a complete characterisation of (ψ2 − γ1)- perfect graphs describing the set of its forbidden induced subgraphs and providing the explicit characterisation of the structure of graphs belonging...

The i-chords of cycles and paths

Terry A. McKee (2012)

Discussiones Mathematicae Graph Theory

An i-chord of a cycle or path is an edge whose endpoints are a distance i ≥ 2 apart along the cycle or path. Motivated by many standard graph classes being describable by the existence of chords, we investigate what happens when i-chords are required for specific values of i. Results include the following: A graph is strongly chordal if and only if, for i ∈ {4,6}, every cycle C with |V(C)| ≥ i has an (i/2)-chord. A graph is a threshold graph if and only if, for i ∈ {4,5}, every path P with |V(P)|...

Currently displaying 1 – 20 of 22

Page 1 Next