Clique-critical graphs: Maximum size and recognition

The clique graph of G, K (G), is the intersection graph of the family of cliques (maximal complete sets) of G. Clique-critical graphs were defined as those whose clique graph changes whenever a vertex is removed. We prove that if G has m edges then any clique-critical graph in K-1 (G) has at most 2m vertices, which solves a question posed by Escalante and Toft [On clique-critical graphs, J. Combin. Theory B 17 (1974) 170-182]. The proof is based on a restatement of their characterization of clique-critical graphs. Moreover, the bound is sharp. We also show that the problem of recognizing clique-critical graphs is NP-complete.Facultad de Ciencias Exacta

A note on path domination

We study domination between different types of walks connecting two non-adjacent vertices u and v of a graph (shortest paths, induced paths, paths, tolled walks). We succeeded in characterizing those graphs in which every uv-walk of one particular kind dominates every uv-walk of other specific kind. We thereby obtained new characterizations of standard graph classes like chordal, interval and superfragile graphs.Facultad de Ciencias Exacta

Finding intersection models: From chordal to Helly circular-arc graphs

Every chordal graph G admits a representation as the intersection graph of a family of subtrees of a tree. A classic way of finding such an intersection model is to look for a maximum spanning tree of the valuated clique graph of G. Similar techniques have been applied to find intersection models of chordal graph subclasses as interval graphs and path graphs. In this work, we extend those methods to be applied beyond chordal graphs: we prove that a graph G can be represented as the intersection of a Helly separating family of graphs belonging to a given class if and only if there exists a spanning subgraph of the clique graph of G satisfying a particular condition. Moreover, such a spanning subgraph is characterized by its weight in the valuated clique graph of G. The specific case of Helly circular-arc graphs is treated. We show that the canonical intersection models of those graphs correspond to the maximum spanning cycles of the valuated clique graph.Facultad de Ciencias Exacta

On asteroidal sets in chordal graphs

We analyze the relation between three parameters of a chordal graph G: the number of non-separating cliques nsc(G), the asteroidal number an(G) and the leafage l(G). We show that an(G) is equal to the maximum value of nsc(H) over all connected induced subgraphs H of G. As a corollary, we prove that if G has no separating simplicial cliques then an(G)=l(G). A graph G is minimal k-asteroidal if an(G)=k and an(H)3; for k=3 it is the family described by Lekerkerker and Boland to characterize interval graphs. We prove that, for every minimal k-asteroidal chordal graph, all the above parameters are equal to k. In addition, we characterize the split graphs that are minimal k-asteroidal and obtain all the minimal 4-asteroidal split graphs. Finally, we applied our results on asteroidal sets to describe the clutters with k edges that are minor-minimal in the sense that every minor has less than k edges.Facultad de Ciencias Exacta

Finding intersection models: From chordal to Helly circular-arc graphs

Every chordal graph G admits a representation as the intersection graph of a family of subtrees of a tree. A classic way of finding such an intersection model is to look for a maximum spanning tree of the valuated clique graph of G. Similar techniques have been applied to find intersection models of chordal graph subclasses as interval graphs and path graphs. In this work, we extend those methods to be applied beyond chordal graphs: we prove that a graph G can be represented as the intersection of a Helly separating family of graphs belonging to a given class if and only if there exists a spanning subgraph of the clique graph of G satisfying a particular condition. Moreover, such a spanning subgraph is characterized by its weight in the valuated clique graph of G. The specific case of Helly circular-arc graphs is treated. We show that the canonical intersection models of those graphs correspond to the maximum spanning cycles of the valuated clique graph.Facultad de Ciencias Exacta

El operador clique y los grafos planares

Se llama completo de un grafo a un conjunto de vértices adyacentes entre sí; si un completo es maximal con respecto a la inclusión, se dice que es un clique del grafo. Los cliques son estructuras especiales que naturalmente han despertado interés desde el mismo inicio de la Teoría de Grafos. Varios problemas famosos, como por ejemplo el problema de coloración de un grafo, o el problema de satisfabilidad de una fórmula lógica, se han vinculado y formulado en términos de los cliques de un grafo. Por otro lado, existe una gama de problemas motivados en el propio estudio de los cliques de un grafo. Particularmente haremos foco en el estudio del grafo que muestra la relación de intersección entre estos cliques: el grafo clique. Dado un grafo G obtenemos el grafo clique de G considerando un vértice por cada clique de G y haciendo dos vértices adyacentes si los correspondientes cliques tienen intersección no vacía. De esta simple definición surgen inmediatamente varias preguntas; las siguientes tres son las que han dado origen a las tres principales líneas de investigación: ¿Todo grafo es el grafo clique de algún grafo? Dada una clase particular de grafos, ¿cómo es la clase formada por los grafos clique de los grafos dados? El proceso, que partiendo de un grafo dado obtiene iterativamente el grafo clique del grafo clique, ¿es convergente o genera una secuencia infinita de distintos grafos?Facultad de Ciencias Exacta

Notas de álgebra y matemática discreta

El lector encontrará en este libro los temas comprendidos en la primera mitad del programa de la asignatura Álgebra que se dicta en la Facultad de Ciencias Exactas de la Universidad Nacional de La Plata para alumnos de las Licenciaturas en Matemática, Física, Astronomía y Geofísica, así como también para alumnos del Profesorado de Matemática. La profundidad con que son tratados los distintos puntos del temario y el orden en que se presentan están en concordancia con los requerimientos de dicha cátedra.Facultad de Ciencias Exacta

A note on path domination

We study domination between different types of walks connecting two non-adjacent vertices u and v of a graph (shortest paths, induced paths, paths, tolled walks). We succeeded in characterizing those graphs in which every uv-walk of one particular kind dominates every uv-walk of other specific kind. We thereby obtained new characterizations of standard graph classes like chordal, interval and superfragile graphs.Facultad de Ciencias Exacta

T-Pebbling in k-connected graphs with a universal vertex

The t-pebbling number is the smallest integer m so that any initially distributed supply of m pebbles can place t pebbles on any target vertex via pebbling moves. The 1-pebbling number of diameter 2 graphs is well-studied. Here we investigate the t-pebbling number of diameter 2 graphs under the lens of connectivity.Fil: Alcón, Liliana Graciela. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Departamento de Matemáticas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata; ArgentinaFil: Gutierrez, Marisa. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Departamento de Matemáticas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata; ArgentinaFil: Hulbert, Glenn. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Departamento de Matemáticas; Argentin

Helly EPT graphs on bounded degree trees : Characterization and recognition

The edge-intersection graph of a family of paths on a host tree is called an EPT graph. When the tree has maximum degree h, we say that the graph is [h, 2, 2]. If, in addition, the family of paths satisfies the Helly property, then the graph is Helly [h, 2, 2]. In this paper, we present a family of EPT graphs called gates which are forbidden induced subgraphs for [h, 2, 2] graphs. Using these we characterize by forbidden induced subgraphs the Helly [h, 2, 2] graphs. As a byproduct we prove that in getting a Helly EPT -representation, it is not necessary to increase the maximum degree of the host tree. In addition, we give an efficient algorithm to recognize Helly [h, 2, 2] graphs based on their decomposition by maximal clique separators.Departamento de Matemátic