Cette thèse présente des résultats obtenus dans deux domaines : la théorie des langages, et la théorie des graphes. En théorie des langages, on s’intéresse à des problèmes de caractérisation de classes de langages réguliers. Le problème générique consiste à déterminer si un langage régulier donné peut être défini dans un certain formalisme. Les méthodes actuelles font intervenir un problème plus général appelé séparation. On présente ici deux types de contributions : une généralisation d’un résultat de décidabilité au cadre des langages de mots infinis, ainsi que des bornes inférieures pour la complexité du problème de séparation. En théorie des graphes, on considère le problème classique de coloration de graphes, où on cherche à attribuer des couleurs aux sommets d’un graphe de sorte que les sommets adjacents reçoivent des couleurs différentes, le but étant d’utiliser le moins de couleurs possible. Dans le cas des graphes peu denses, la méthode de déchargement est un atout majeur. Elle a notamment joué un rôle décisif dans la preuve du théorème des quatre couleurs. Cette méthode peut être vue comme une construction non conventionnelle d’un schéma de preuve par induction, spécifique à la classe de graphes et à la propriété considérées, et où la validité du schéma est rarement immédiate. On utilise des variantes de la méthode de déchargement pour étudier deux types de problèmes de coloration. / In this thesis, we present results obtained in two fields: formal language theory and graph theory. In formal language theory, we consider some problems of characterization of classes of regular languages. The generic problem consists in determining whether a given regular language can be defined in a fixed formalism. The current approaches use a more general problem called separation. We present here two types of contributions: a generalization of a decidability result to the setting of infinite words, together with lower bounds for the complexity of the separation problem. In graph theory, we consider the classical problem of graph coloring, where we assign colors to vertices of a graph in such a way that two adjacent vertices receive different colors. The goal is to use the fewest colors. When the graphs are sparse, a crucial tool for this is the discharging method. It is most notably decisive in the proof of the Four-Color Theorem. This method can be seen as an unconventional construction of an inductive proof scheme, specific to the considered problem and graph class, where arguing the validity of the scheme is rarely immediate. We use variants of the discharging method to study two types of coloring problems.

Made available in DSpace on 2014-06-11T19:27:55Z (GMT). No. of bitstreams: 0 Previous issue date: 2007-01-17Bitstream added on 2014-06-13T20:17:29Z : No. of bitstreams: 1 lozano_d_me_sjrp.pdf: 1183878 bytes, checksum: 4a6bb33915f7d1702bf0df3808789aa1 (MD5) / Secretaria de Educação do Estado de São Paulo / O objetivo desse trabalho é apresentar o problema de coloração em grafos sob diferentes perspectivas. Caracterizamos o polinômio cromático de um grafo e enunciamos algumas de suas propriedades. Apresentamos duas formulações matemáticas para o problema de coloração de vértices e um método de solução para cada formulação. Apresentamos e discutimos propostas de atividades para o desenvolvimento de uma Oficina de Coloração para alunos do Ensino Médio e Fundamental. / In this work the graph coloring problem was presented under di erent perspectives. We define the chromatic polynomials of a graph and describe some of its properties. Furthermore, two solution methods for the vertex coloring problem, through integer programming formulation, has been presented. We propose and discuss some activities for the development of a Workshop for students of secondary school.

Orientador: Celia Picinin de Mello / Tese (doutorado) - Universidade Estadual de Campinas , Instituto de Computação / Made available in DSpace on 2018-08-06T12:11:33Z (GMT). No. of bitstreams: 1 Campos_ChristianeNeme_D.pdf: 1048367 bytes, checksum: e8270db6704873ddaf2043927ca93e99 (MD5) Previous issue date: 2006 / Doutorado / Teoria dos Grafos / Doutor em Ciência da Computação

Submitted by Renata Lopes (renatasil82@gmail.com) on 2017-09-21T18:30:26Z No. of bitstreams: 1 lilianerodriguesdealmeida.pdf: 4991501 bytes, checksum: 0d4dbf0a69e24ecbaec4ff6581381709 (MD5) / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2017-09-22T15:22:19Z (GMT) No. of bitstreams: 1 lilianerodriguesdealmeida.pdf: 4991501 bytes, checksum: 0d4dbf0a69e24ecbaec4ff6581381709 (MD5) / Made available in DSpace on 2017-09-22T15:22:19Z (GMT). No. of bitstreams: 1 lilianerodriguesdealmeida.pdf: 4991501 bytes, checksum: 0d4dbf0a69e24ecbaec4ff6581381709 (MD5) Previous issue date: 2017-02-23 / FAPEMIG - Fundação de Amparo à Pesquisa do Estado de Minas Gerais / Este trabalho propõe um método de reposicionamento de polilinhas em 2D que representam curvas cilíndricas para manter a distância entre os segmentos de reta com pelo menos e unidades mais os raios de dois cilindros quaisquer, cada um associado a uma polilinha. A abordagem depende da construção de um grafo que representa os pontos que violam uma distância mínima, reduzindo o problema de remoção de contatos ao problema de coloração de grafos. Uma vez construído, o grafo é colorido usando uma heurística para encontrar quais vértices podem estar no mesmo plano. O número final de cores indica o número de planos na terceira dimensão necessários para resolver os contatos. Propõe-se também duas abordagens para calcular os deslocamentos dos vértices a partir dos grafo e das cores computadas, ambas projetadas para obter florestas com a soma de deslocamentos mínima. Os resultados mostram a eficiência da construção do grafo, da coloração do grafo e do mapeamento de cor em planos. Aplica-se o método proposto e as duas abordagens de deslocamento no problema de desentrelaçamento de florestas de polilinhas que representam nanotubos de carbono. O número de contatos cai significativamente depois da aplicação do método mesmo em florestas de tubos densas e com vários contatos. / This work proposes a method to reposition of 2D polylines representing cylindrical curves in order to keep the distance between line segments with at least c unities plus the radii of any two cylinders, each associated with a polyline. Our approach relies on the construction of a graph representing the points violating a minimum distance, reducing the contact removal problem to a graph coloring problem. Once constructed, the graph is colored using a heuristic to find out which vertices can be in the same plane. The final number of colors indicates the number of planes in third dimension needed to solve contacts. We also propose two approaches to compute vertex displacements from the computed graph and colors, both designed to obtain forests with minimum sum of displacements. Results show the efficiency of the graph construction, graph coloring and color to plane mappings. We apply the proposed method and the two displacement approaches on the problem of untangling forests of polylines representing carbon nanotubes. The number of contacts drops significantly after applying our method even in dense forests of tubes with numerous contacts.

For a finite relational structure A, the Constraint Satisfaction Problem with template A, or CSP(A), is the problem of deciding whether an input relational structure X admits a homomorphism to A. The CSP dichotomy conjecture of Feder and Vardi states that for any A, CSP(A) is either in P or NP-complete. In the first part we present the algebraic approach to CSP and summarize known results about CSP for digraphs, also known as the H-coloring problem. In the second part we study a class of oriented trees called special polyads. Using the algebraic approach we confirm the dichotomy conjecture for special polyads. We provide a finer description of the tractable cases and give a construction of a special polyad T such that CSP(T) is tractable, but T does not have width 1 and admits no near-unanimity polymorphisms.

This thesis consists of three parts devoted to graph labeling, hereditary graph classes, and parameterized complexity. Packing coloring, originally Broadcasting Chromatic number, assigns natural numbers to vertices such that vertices with the same label are in distance at least the value of the label. This problem is motivated by the assignment of frequencies to the transmitters. We improve hardness on chordal graphs. We proof that packing coloring on chordal graphs with diameter 3 is very hard to approximate. Moreover, we discuss several positive results on interval graphs and on related structural graph parameters. Hereditary graph classes are preserved under vertex deletion. We study graphs that do not contain an induced subgraph H. We prove that 3-coloring is polynomial-time solvable for (P3 + P4)-free and (P2 + P5)-free graphs and thus we have solved the last open cases for the problem on H-free graphs where H has up to 7 vertices. Fair problems are a modification of graph deletion problems, where, instead of minimizing the size of the solution, the aim is to minimize the maximum number of neighbors in the deleted set. We show that those problems can be solved in FPT time for an MSO1 formula parameterized by the size of the formula and the twin cover of the graph. Moreover, we define a basic...

A star edge coloring is any edge coloring which is both proper and contains no cycles or path of length four which are bicolored, and the star chromatic index of a graph is the smallest number of colors for which that graph can be star edge colored. Star edge coloring is a relatively new field in graph theory, and very little is known regarding upper bounds of the star chromatic index of most graph types, one of these families being multipartite graphs. We investigate a method for obtaining upper bounds on the star chromatic index of complete multipartite graphs. The basic idea is to decompose such graphs into smaller complete bipartite graphs and applying known upper bounds for such graphs.This method has also been implemented and we present a hypothesis based on simulations.

Quantum annealing (QA) is an optimization process in quantum computing similar to the probabilistic metaheuristic simulated annealing (SA). The QA process involves encoding an optimization problem into an energy landscape, which it then traverses in search for the point of minimal energy representing the global optimal state. In this thesis two different implementations of QA are examined, one run on a binary quadratic model (BQM) and one on a discrete quadratic model (DQM). These are then compared to their traditional counterpart: SA, in terms of performance and accuracy when solving the graph coloring problem (GCP). Regarding performance, the results illustrate how SA outperforms both QA implementations. However, it is apparent that these slower execution times are mostly due to various overhead costs that appear because of limited hardware. When only looking at the quantum annealing part of the process, it is about a hundred times faster than the SA process. When it comes to accuracy, both the DQM-implementation of QA and SA provided results of high quality, whereas the BQM-implementation performed notably worse, both by often not finding the optimal values and by sometimes returning invalid results. / Quantum annealing (QA) är en kvantbaserad optimeringsprocess som liknar den probabilistiska metaheuristiken simulated annealing (SA). QA går ut på att konvertera ett optimeringsproblem till ett energilandskap, som sedan navigeras för att hitta punkten med lägst energi, vilket då motsvarar den optimala lösningen på problemet. I denna uppsats undersöks två olika implementationer av QA: en som använder en binary quadratic model (BQM) och en som använder en discrete quadratic model (DQM). Dessa två implementationerna jämförs med deras traditionella motsvarighet: SA, utifrån både prestanda och korrekthet vid lösning av graffärgningsproblemet (GCP). När det gäller prestanda visar resultaten att SA är snabbare än båda QA implementationerna. Samtidigt är det tydligt att denna prestandaskillnad framförallt beror på diverse förberedelser innan exkueringen startar på kvantdatorn, vilka är krävande på grund av olika hårdvarubegränsningar. Om man endast betraktar kvantprocesserna visar vår studie att QA implementationerna är ungefär hundra gånger snabbare än SA. Gällande korrekthet gav både DQM-implementationen av QA och SA resultat av hög kvalitet medan BQM-implementationen presterade betydligt sämre. Den gjorde detta dels genom att inte skapa optimala resultat och genom att returnera otillåtna lösningar.

A Probabilistically Checkable Proof (PCP) of a mathematical statement is a proof written in a special manner that allows for efficient probabilistic verification. The celebrated PCP Theorem states that for every family of statements in NP, there is a probabilistic verification procedure that checks the validity of a PCP proof by reading only 3 bits from it. This landmark theorem, and the works leading up to it, laid the foundation for many subsequent works in computational complexity theory, the most prominent among them being the study of inapproximability of combinatorial optimization problems. This thesis focuses on a broad class of combinatorial optimization problems called Constraint Satisfaction Problems (CSPs). In an instance of a CSP problem of arity k, we are given a set of variables taking values from some finite domain, and a set of constraints each involving a subset of at most k variables. The goal is to find an assignment that simultaneously satisfies as many constraints as possible. An alternative formulation of the goal that is commonly used is Gap-CSP, where the goal is to decide whether a CSP instance is satisfiable or far from satisfiable, where the exact meaning of being far from satisfiable varies depending on the problems.We first study Boolean CSPs, where the domain of the variables is {0,1}. The main question we study is the hardness of distinguishing satisfiable Boolean CSP instances from those for which no assignment satisfies more than some epsilon fraction of the constraints. Intuitively, as the arity increases, the CSP gets more complex and thus the hardness parameter epsilon should decrease. We show that for Boolean CSPs of arity k, it is NP-hard to distinguish satisfiable instances from those that are at most 2^{~O(k^{1/3})}/2^k-satisfiable. We also study coloring of graphs and hypergraphs. Given a graph or a hypergraph, a coloring is an assignment of colors to vertices, such that all edges or hyperedges are non-monochromatic. The gap problem is to distinguish instances that are colorable with a small number of colors, from those that require a large number of colors. For graphs, we prove that there exists a constant K_0&gt;0, such that for any K &gt;= K_0, it is NP-hard to distinguish K-colorable graphs from those that require 2^{Omega(K^{1/3})} colors. For hypergraphs, we prove that it is quasi-NP-hard to distinguish 2-colorable 8-uniform hypergraphs of size N from those that require 2^{(log N)^{1/4-o(1)}} colors. In terms of techniques, all these results are based on constructions of PCPs with perfect completeness, that is, PCPs where the probabilistic proof verification procedure always accepts a correct proof. Not only is this a very natural property for proofs, but it can also be an essential requirement in many applications. It has always been particularly challenging to construct PCPs with perfect completeness for NP statements due to limitations in techniques. Our improved hardness results build on and extend many of the current approaches. Our Boolean CSP result and GraphColoring result were proved by adapting the Direct Sum of PCPs idea by Siu On Chan to the perfect completeness setting. Our proof for hypergraph coloring hardness improves and simplifies the recent work by Khot and Saket, in which they proposed the notion of superposition complexity of CSPs. / Ett probabilistiskt verifierbart bevis (eng: Probabilistically Checkable Proof, PCP) av en matematisk sats är ett bevis skrivet på ett speciellt sätt vilket möjliggör en effektiv probabilistisk verifiering. Den berömda PCP-satsen säger att för varje familj av påståenden i NP finns det en probabilistisk verifierare som kontrollerar om en PCP bevis är giltigt genom att läsa endast 3 bitar från det. Denna banbrytande sats, och arbetena som ledde fram till det, lade grunden för många senare arbeten inom komplexitetsteorin, framförallt inom studiet av approximerbarhet av kombinatoriska optimeringsproblem. I denna avhandling fokuserar vi på en bred klass av optimeringsproblem i form av villkorsuppfyllningsproblem (engelska ``Constraint Satisfaction Problems'' CSPs). En instans av ett CSP av aritet k ges av en mängd variabler som tar värden från någon ändlig domän, och ett antal villkor som vart och ett beror på en delmängd av högst k variabler. Målet är att hitta ett tilldelning av variablerna som samtidigt uppfyller så många som möjligt av villkoren. En alternativ formulering av målet som ofta används är Gap-CSP, där målet är att avgöra om en CSP-instans är satisfierbar eller långt ifrån satisfierbar, där den exakta innebörden av att vara ``långt ifrån satisfierbar'' varierar beroende på problemet.Först studerar vi booleska CSPer, där domänen är {0,1}. Den fråga vi studerar är svårigheten av att särskilja satisfierbara boolesk CSP-instanser från instanser där den bästa tilldelningen satisfierar högst en andel epsilon av villkoren. Intuitivt, när ariten ökar blir CSP mer komplexa och därmed bör svårighetsparametern epsilon avta med ökande aritet. Detta visar sig vara sant och ett första resultat är att för booleska CSP av aritet k är det NP-svårt att särskilja satisfierbara instanser från dem som är högst 2^{~O(k^{1/3})}/2^k-satisfierbara. Vidare studerar vi färgläggning av grafer och hypergrafer. Givet en graf eller en hypergraf, är en färgläggning en tilldelning av färger till noderna, så att ingen kant eller hyperkant är monokromatisk. Problemet vi analyserar är att särskilja instanser som är färgbara med ett litet antal färger från dem som behöver många färger. För grafer visar vi att det finns en konstant K_0&gt;0, så att för alla K &gt;= K_0 är det NP-svårt att särskilja grafer som är K-färgbara från dem som kräver minst 2^{Omega(K^{1/3})} färger. För hypergrafer visar vi att det är kvasi-NP-svårt att särskilja 2-färgbara 8-likformiga hypergrafer som har N noder från dem som kräv minst 2^{(log N)^{1/4-o(1)}} färger. Samtliga dessa resultat bygger på konstruktioner av PCPer med perfekt fullständighet. Det vill säga PCPer där verifieraren alltid accepterar ett korrekt bevis. Inte bara är detta en mycket naturlig egenskap för PCPer, men det kan också vara ett nödvändigt krav för vissa tillämpningar. Konstruktionen av PCPer med perfekt fullständighet för NP-påståenden ger tekniska komplikationer och kräver delvis utvecklande av nya metoder. Vårt booleska CSPer resultat och vårt Färgläggning resultat bevisas genom att anpassa ``Direktsumman-metoden'' introducerad av Siu On Chan till fallet med perfekt fullständighet. Vårt bevis för hypergraffärgningssvårighet förbättrar och förenklar ett färskt resultat av Khot och Saket, där de föreslog begreppet superpositionskomplexitet av CSP. / <p>QC 20150916</p>

Dans cette thèse, nous étudions divers problèmes dont l'objectif est de gérer la croissance d'internet plus efficacement. En effet celle-ci est très vive : 41% pour le pic en 2012. Afin de répondre aux défis posés par cette évolution aux divers acteurs du réseau, des protocoles de gestion et de communication plus intelligents sont nécessaires. Les protocoles de l'Internet furent conçus, point à point. Or, la part de la diffusion de média dans le trafic est prépondérante et en hausse tendancielle, et des projections indiquent qu'en 2016 80-90% du trafic sera engendré par de la diffusion vidéo. Cette divergence entraîne des inefficacités car les données parcourent plusieurs fois le réseau. Dans cette thèse, nous étudions comment tempérer cette inefficacité. Nos contributions sont organisées selon les couches et les phases de déploiement du réseau. Nous étudions le placement de caches lors de la conception du réseau. Ensuite, pour la gestion d'un réseau, nous regardons quand placer des appareils en veille, en utilisant un mécanisme de cache et en coopération avec des réseaux de distribution. Puis, au niveau de la couche application, nous étudions un problème de maintenance d'arbres équilibrés pour la diffusion de média. Enfin, nous analysons la probabilité de survie de données dans un système de sauvegarde distribuée. Notre travail se fonde à la fois sur des méthodes théoriques (Chaînes de Markov, Programmation Linéaire), mais aussi sur des outils empiriques tels que la simulation et l'expérimentation. / In this thesis we study multiple approaches to efficiently accommodating for the future growth of the Internet. The exponential growth of Internet traffic, reported to be as high as 41% in peak throughput in 2012 alone, continues to pose challenges to all interested parties. Therefore, to accommodate the growth, smart management and communication protocols are needed. The basic protocols of the Internet are point-to-point in nature. However, the traffic is largely broadcasting, with projections stating that as much as 80-90% of it will be video by 2016. This discrepancy leads to inefficiency, where multiple copies of essentially the same messages travel in parallel through the same links. In this thesis we study multiple approaches to mitigating this inefficiency. The contributions are organized by layers and phases of the network life. We look into optimal cache provisioning during network design. Next, we move to managing an existing network. We look into putting devices to sleep mode, using caching and cooperation with Content Distribution Networks. In the application layer, we look into maintaining balanced trees for media broadcasting. Finally, we analyze data survivability in a distributed backup system, which can reduce network traffic by putting the backups closer to the client than if using a data center. Our work is based on both theoretical methods, like Markov chains and linear programming, as well as empirical tools, like simulation and experimentation.