Rotulações próprias por gap : variantes de arestas e de vértices

Orientadores: Christiane Neme Campos, Rafael Crivellari Saliba SchoueryDissertação (mestrado) - Universidade Estadual de Campinas, Instituto de ComputaçãoResumo: Uma rotulação própria é uma atribuição de valores numéricos aos elementos de um grafo, que podem ser seus vértices, arestas ou ambos, de modo a obter - usando certas funções matemáticas sobre o conjunto de rótulos - uma coloração dos vértices do grafo tal que nenhum par de vértices adjacentes receba a mesma cor. Este texto aborda o problema da rotulação própria por gap em suas versões de arestas e de vértices. Na versão de arestas, um vértice de grau pelo menos dois tem sua cor definida como a maior diferença, i.e. o maior gap, entre os rótulos de suas arestas incidentes; já na variante de vértices, o gap é definido pela maior diferença entre os rótulos dos seus vértices adjacentes. Para vértices de grau um, sua cor é dada pelo rótulo da sua aresta incidente, no caso da versão de arestas, e pelo rótulo de seu vértice adjacente, no caso da versão de vértices. Finalmente, vértices de grau zero recebem cor um. O menor número de rótulos para o qual um grafo admite uma rotulação própria por gap de arestas vértices é chamado edge-gap (vertex-gap) number. Neste trabalho, apresentamos um breve histórico das rotulações próprias por gap e os resultados obtidos para as duas versões do problema. Estudamos o edge-gap e o vertex-gap numbers para as famílias de ciclos, coroas, rodas, grafos unicíclicos e algumas classes de snarks. Adicionalmente, na versão de vértices, investigamos a família de grafos cúbicos bipartidos hamiltonianos, desenvolvendo diversas técnicas de rotulação para grafos nesta classe. Em uma abordagem relacionada, provamos resultados de complexidade para a família dos grafos subcúbicos bipartidos. Além disso, demonstramos propriedades estruturais destas rotulações de vértices por gap e estabelecemos limitantes inferiores e superiores justos para o vertex-gap number de grafos arbitrários. Mostramos, ainda, que nem todos os grafos admitem uma rotulação de vértices por gap, exibindo duas famílias infinitas que não admitem tal rotulação. A partir dessas classes, definimos um novo parâmetro chamado de gap-strength, referente ao menor número de arestas que precisam ser removidas de um grafo de modo a obter um novo grafo que é gap-vértice-rotulável. Estabelecemos um limitante superior para o gap-strength de grafos completos e apresentamos evidências de que este valor pode ser um limitante inferiorAbstract: A proper labelling is an assignment of numerical values to the elements of a graph, which can be vertices, edges or both, so as to obtain - through the use of mathematical functions over the set of labels - a vertex-colouring of the graph such that every pair of adjacent vertices receives different colours. This text addresses the proper gap-labelling problem in its edge and vertex variants. In the former, a vertex of degree at least two has its colour defined by the largest difference, or gap, among the labels of its incident edges; in the vertex variant, the gap is defined by the largest difference among the labels of its adjacent vertices. For a degree-one vertex, its colour is defined by the label of its incident edge, in the edge version, and by the label of its adjacent vertex, in the vertex variant. Finally, degree-zero vertices receive colour one. The least number of labels for which a graph admits a proper gap-labelling of its edges (vertices) is called the edge-gap (vertex-gap) number. In this work, we present a brief history of proper gap-labellings and our results for both versions of the problem. We study the edge-gap and vertex-gap numbers for the families of cycles, crowns, wheels, unicyclic graphs and some classes of snarks. Additionally, in the vertex version, we investigate the family of cubic bipartite hamiltonian graphs and develop several labelling techniques for graphs in this class. In a related approach, we prove hardness results for the family of subcubic bipartite graphs. Also, we demonstrate structural properties of gap-vertex-labelable graphs and establish tight lower and upper bounds for the vertex-gap number of arbitrary graphs. We also show that not all graphs admit a proper gap-labelling, exhibiting two infinite families of graphs for which no such vertex-labelling exists. Thus, we define a new parameter called the gap-strength of graphs, which is the least number of edges that have to be removed from a graph so as to obtain a new, gap-vertex-labelable graph. We establish an upper bound for the gap-strength of complete graphs and argue that this value can also be used as a lower boundMestradoCiência da ComputaçãoMestre em Ciência da ComputaçãoCAPE

Linear-Time Algorithms for Edge-Based Problems

There is a dearth of algorithms that deal with edge-based problems in trees, specifically algorithms for edge sets that satisfy a particular parameter. The goal of this thesis is to create a methodology for designing algorithms for these edge-based problems. We will present a variant of the Wimer method [Wimer et al. 1985] [Wimer 1987] that can handle edge properties. We call this variant the Wimer edge variant. The thesis is divided into three sections, the first being a chapter devoted to defining and discussing the Wimer edge variant in depth, showing how to develop an algorithm using this variant, and an example of this process, including a run of an algorithm developed using this method. The second section involves algorithms developed using the Wimer edge variant. We will provide algorithms for a variety of edge parameters, including four different matching parameters (connected, disconnected, induced and 2-matching), three different domination parameters (edge, total edge and edge-vertex) and two covering parameters (edge cover and edge cover irredundance). Each of these algorithms are discussed in detail and run in linear time. The third section involves an attempt to characterize the Wimer edge variant. We show how the variant can be applied to three classes of graphs: weighted trees, unicyclic graphs and generalized series-parallel graphs. For each of these classes, we detail what adaptations are required (if any) and design an algorithm, including showing a run on an example graph. The fourth chapter is devoted to a discussion of what qualities a parameter has to have in order to be likely to have a solution using the Wimer edge variant. Also in this chapter we discuss classes of graphs that can utilize the Wimer edge variant. Other topics discussed in this thesis include a literature review, and a discussion of future work. There are plenty of options for future work on this topic, which hopefully this thesis can inspire. The intent of this thesis is to provide the foundation for future algorithms and other work in this area

Revolutionaries and spies: Spy-good and spy-bad graphs

We study a game on a graph $G$ played by $r$ {\it revolutionaries} and $s$ {\it spies}. Initially, revolutionaries and then spies occupy vertices. In each subsequent round, each revolutionary may move to a neighboring vertex or not move, and then each spy has the same option. The revolutionaries win if $m$ of them meet at some vertex having no spy (at the end of a round); the spies win if they can avoid this forever. Let $\sigma(G,m,r)$ denote the minimum number of spies needed to win. To avoid degenerate cases, assume |V(G)|\ge r-m+1\ge\floor{r/m}\ge 1. The easy bounds are then \floor{r/m}\le \sigma(G,m,r)\le r-m+1. We prove that the lower bound is sharp when $G$ has a rooted spanning tree $T$ such that every edge of $G$ not in $T$ joins two vertices having the same parent in $T$. As a consequence, \sigma(G,m,r)\le\gamma(G)\floor{r/m}, where $\gamma(G)$ is the domination number; this bound is nearly sharp when $\gamma(G)\le m$. For the random graph with constant edge-probability $p$, we obtain constants $c$ and $c'$ (depending on $m$ and $p$) such that $\sigma(G,m,r)$ is near the trivial upper bound when $r<c\ln n$ and at most $c'$ times the trivial lower bound when $r>c'\ln n$. For the hypercube $Q_d$ with $d\ge r$, we have $\sigma(G,m,r)=r-m+1$ when $m=2$, and for $m\ge 3$ at least $r-39m$ spies are needed. For complete $k$-partite graphs with partite sets of size at least $2r$, the leading term in $\sigma(G,m,r)$ is approximately $\frac{k}{k-1}\frac{r}{m}$ when $k\ge m$. For $k=2$, we have \sigma(G,2,r)=\bigl\lceil{\frac{\floor{7r/2}-3}5}\bigr\rceil and \sigma(G,3,r)=\floor{r/2}, and in general $\frac{3r}{2m}-3\le \sigma(G,m,r)\le\frac{(1+1/\sqrt3)r}{m}$.Comment: 34 pages, 2 figures. The most important changes in this revision are improvements of the results on hypercubes and random graphs. The proof of the previous hypercube result has been deleted, but the statement remains because it is stronger for m<52. In the random graph section we added a spy-strategy resul

Rainbow Colorings in Graphs

In this thesis, we deal with rainbow colorings of graphs. We engage not with the rainbow connection number but with counting of rainbow colorings in graphs with k colors. We introduce the rainbow polynomial and prove some results for some special graph classes. Furthermore, we obtain bounds for the rainbow polynomial. In addition, we define some edge colorings related to the rainbow coloring, like the s-rainbow coloring and the 2-rainbow coloring. For this edge colorings, polynomials are defined and we prove some basic properties for this polynomials and present some formulas for the calculation in special graph classes. In addition, we consider in this thesis counting problems related to the rainbow coloring like rainbow pairs and rainbow dependent sets. We introduce polynomials for this counting problems and present some general properties and formulas for special graph classes