26 research outputs found
Sobre a complexidade de coloração mista
National audienceGrafos mistos sĂŁo estruturas matemĂĄticas que mesclam caracterĂsticas de grafos direcionados e nĂŁo-direcionados. Formalmente, um grafo misto pode ser deïŹnido por uma tripla GM = (V, A, E), onde V , A e E representam, respectivamente, um conjunto de vĂ©rtices, de arcos e de arestas. Uma k-coloração mista de GM = (V, A, E) Ă© função c : V â {0, . . . , k â 1} tal que c(u) < c(v), se (u, v) â A, e c(u) = c(v), se [u, v] â E. O problema de Coloração Mista consiste em determinar o nĂșmero cromĂĄtico misto de GM , denotado por ÏM (GM ), que Ă© o menor inteiro k tal que GM admite uma k-coloração mista. Esse problema modela variaçÔes de problemas de escalonamento que consideram simultaneamente restriçÔes de precedĂȘncia e de compartilhamento de recursos. Neste trabalho, mostramos que Coloração Mista Ă© N P -difĂcil para as classes dos grafos cordais, dos grafos linha de grafos bipartidos e dos grafos linha de grafos periplanares
Sobre a complexidade de coloração mista
National audienceGrafos mistos sĂŁo estruturas matemĂĄticas que mesclam caracterĂsticas de grafos direcionados e nĂŁo-direcionados. Formalmente, um grafo misto pode ser deïŹnido por uma tripla GM = (V, A, E), onde V , A e E representam, respectivamente, um conjunto de vĂ©rtices, de arcos e de arestas. Uma k-coloração mista de GM = (V, A, E) Ă© função c : V â {0, . . . , k â 1} tal que c(u) < c(v), se (u, v) â A, e c(u) = c(v), se [u, v] â E. O problema de Coloração Mista consiste em determinar o nĂșmero cromĂĄtico misto de GM , denotado por ÏM (GM ), que Ă© o menor inteiro k tal que GM admite uma k-coloração mista. Esse problema modela variaçÔes de problemas de escalonamento que consideram simultaneamente restriçÔes de precedĂȘncia e de compartilhamento de recursos. Neste trabalho, mostramos que Coloração Mista Ă© N P -difĂcil para as classes dos grafos cordais, dos grafos linha de grafos bipartidos e dos grafos linha de grafos periplanares
Optimal k-fold colorings of webs and antiwebs
A k-fold x-coloring of a graph is an assignment of (at least) k distinct
colors from the set {1, 2, ..., x} to each vertex such that any two adjacent
vertices are assigned disjoint sets of colors. The smallest number x such that
G admits a k-fold x-coloring is the k-th chromatic number of G, denoted by
\chi_k(G). We determine the exact value of this parameter when G is a web or an
antiweb. Our results generalize the known corresponding results for odd cycles
and imply necessary and sufficient conditions under which \chi_k(G) attains its
lower and upper bounds based on the clique, the fractional chromatic and the
chromatic numbers. Additionally, we extend the concept of \chi-critical graphs
to \chi_k-critical graphs. We identify the webs and antiwebs having this
property, for every integer k <= 1.Comment: A short version of this paper was presented at the Simp\'osio
Brasileiro de Pesquisa Operacional, Brazil, 201
Stable sets, corner polyhedra and the ChvĂĄtal closure
In this work, we consider a classical formulation of the stable set problem. We characterize its corner polyhedron, i.e. the convex hull of the points satisfying all the constraints except the non-negativity of the basic variables. We show that the non-trivial inequalities necessary to describe this polyhedron can be derived from one row of the simplex tableau as fractional Gomory cuts. It follows in particular that the split closure is not stronger than the ChvĂĄtal closure for the stable set problem. The results are obtained via a characterization of the basis and its inverse in terms of a collection of connected components with at most one cycle
Facets of the polytope of legal sequences
A sequence of vertices in a graph is called a (total) legal dominating sequence if every vertex in the sequence (totally) dominates at least one vertex not dominated by the ones that precedes it, and at the end all vertices of the graph are (totally) dominated. The Grundy (total) domination number of a graph is the size of the largest (total) legal dominating sequence. In this work, we present integer programming formulations for obtaining the Grundy (total) domination number of a graph, we study some aspects of the polyhedral structure of one of them and we test the performance of some new valid inequalities as cuts.Fil: CampĂȘlo, Manoel. Universidade Federal Do CearĂĄ; BrasilFil: Severin, Daniel Esteban. Universidad Nacional de Rosario. Facultad de Ciencias Exactas IngenierĂa y Agrimensura. Escuela de Ciencias Exactas y Naturales. Departamento de MatemĂĄtica; Argentina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; Argentin
An ADD/DROP procedure for the capacitated plant location problem
The capacitated plant location problem with linear transportation costs is considered. Exact rules and heuristics are presented for opening or closing of facilities. A heuristic algorithm based on ADD/DROP strategies is proposed. Procedures are implemented with the help of lower and upper bounds using Lagrangean relaxation. Computational results are presented and comparisons with other algorithms are made