A $(m,n)$-mixed graph is a mixed graph whose edges are assigned one of $m$ colours, and whose arcs are assigned one of $n$ colours. Let $G$ be a $(m,n)$-mixed graph and $\pi=(\alpha,\beta,\gamma_1,\gamma_2,\ldots,\gamma_n)$ be a $(n+2)$-tuple of permutations from $S_m \times S_n \times S_2^n$. We define \emph{switching at a vertex $v$ with respect to $\pi$} as follows. Replace each edge $vw$ of colour $\phi$ by an edge $vw$ of colour $\alpha(\phi)$, and each arc $vx$ of colour $\phi$ by an arc $\gamma_\phi(vx)$ of colour $\beta(\phi)$. In this thesis, we study the complexity of the question: ``Given a $(m,n)$-mixed graph $G$, is there a sequence of switches at vertices of $G$ with respect to the fixed group $\Gamma$ so that the resulting $(m,n)$-mixed graph admits a homomorphism to some $(m,n)$-mixed graph on $2$ vertices?'' We show the following: (1) When restricted to $(m,0)$-mixed graphs $H$ on at most $2$ vertices, the $\Gamma$-switchable homomorphism decision problem is solvable in polynomial time; (2) for each bipartite $(0,n)$-mixed graph $H$, there is a bipartite $(2n,0)$-mixed graph such that the respective $\Gamma$-switchable homomorphism decision problems are polynomially equivalent; (3) For all $(m,n)$-mixed graphs and groups, when $H$ has at most $2$ vertices, the $\Gamma$-switchable homomorphism decision problem is polynomial time solvable; (4) For a yes-instance of the $\Gamma$-switchable homomorphism problem for $(m,0)$-mixed graphs, we can find in quadratic time a sequence of switches on $G$ such that the resulting $(m,0)$-mixed graph admits a homomorphism to $H$. By proving (1)-(4), we show that the $\Gamma$-switchable $2$-colouring problem for $(m,n)$-mixed graphs is solvable in polynomial time for all finite permutation groups $\Gamma$ and provide a step towards a dichotomy theorem for the complexity of the $\Gamma$-switchable homomorphism decision problem. / Graduate

This thesis investigates three areas of the theory of graph homomorphisms: cores of graphs, the homomorphism order, and quantum homomorphisms. A core of a graph X is a vertex minimal subgraph to which X admits a homomorphism. Hahn and Tardif have shown that, for vertex transitive graphs, the size of the core must divide the size of the graph. This motivates the following question: when can the vertex set of a vertex transitive graph be partitioned into sets which each induce a copy of its core? We show that normal Cayley graphs and vertex transitive graphs with cores half their size always admit such partitions. We also show that the vertex sets of vertex transitive graphs with cores less than half their size do not, in general, have such partitions. Next we examine the restriction of the homomorphism order of graphs to line graphs. Our main focus is in comparing this restriction to the whole order. The primary tool we use in our investigation is that, as a consequence of Vizing's theorem, this partial order can be partitioned into intervals which can then be studied independently. We denote the line graph of X by L(X). We show that for all n ≥ 2, for any line graph Y strictly greater than the complete graph Kₙ, there exists a line graph X sitting strictly between Kₙ and Y. In contrast, we prove that there does not exist any connected line graph which sits strictly between L(Kₙ) and Kₙ, for n odd. We refer to this property as being ``n-maximal", and we show that any such line graph must be a core and the line graph of a regular graph of degree n. Finally, we introduce quantum homomorphisms as a generalization of, and framework for, quantum colorings. Using quantum homomorphisms, we are able to define several other quantum parameters in addition to the previously defined quantum chromatic number. We also define two other parameters, projective rank and projective packing number, which satisfy a reciprocal relationship similar to that of fractional chromatic number and independence number, and are closely related to quantum homomorphisms. Using the projective packing number, we show that there exists a quantum homomorphism from X to Y if and only if the quantum independence number of a certain product graph achieves |V(X)|. This parallels a well known classical result, and allows us to construct examples of graphs whose independence and quantum independence numbers differ. Most importantly, we show that if there exists a quantum homomorphism from a graph X to a graph Y, then ϑ̄(X) ≤ ϑ̄(Y), where ϑ̄ denotes the Lovász theta function of the complement. We prove similar monotonicity results for projective rank and the projective packing number of the complement, as well as for two variants of ϑ̄. These immediately imply that all of these parameters lie between the quantum clique and quantum chromatic numbers, in particular yielding a quantum analog of the well known ``sandwich theorem". We also briefly investigate the quantum homomorphism order of graphs.

This thesis investigates three areas of the theory of graph homomorphisms: cores of graphs, the homomorphism order, and quantum homomorphisms. A core of a graph X is a vertex minimal subgraph to which X admits a homomorphism. Hahn and Tardif have shown that, for vertex transitive graphs, the size of the core must divide the size of the graph. This motivates the following question: when can the vertex set of a vertex transitive graph be partitioned into sets which each induce a copy of its core? We show that normal Cayley graphs and vertex transitive graphs with cores half their size always admit such partitions. We also show that the vertex sets of vertex transitive graphs with cores less than half their size do not, in general, have such partitions. Next we examine the restriction of the homomorphism order of graphs to line graphs. Our main focus is in comparing this restriction to the whole order. The primary tool we use in our investigation is that, as a consequence of Vizing's theorem, this partial order can be partitioned into intervals which can then be studied independently. We denote the line graph of X by L(X). We show that for all n ≥ 2, for any line graph Y strictly greater than the complete graph Kₙ, there exists a line graph X sitting strictly between Kₙ and Y. In contrast, we prove that there does not exist any connected line graph which sits strictly between L(Kₙ) and Kₙ, for n odd. We refer to this property as being ``n-maximal", and we show that any such line graph must be a core and the line graph of a regular graph of degree n. Finally, we introduce quantum homomorphisms as a generalization of, and framework for, quantum colorings. Using quantum homomorphisms, we are able to define several other quantum parameters in addition to the previously defined quantum chromatic number. We also define two other parameters, projective rank and projective packing number, which satisfy a reciprocal relationship similar to that of fractional chromatic number and independence number, and are closely related to quantum homomorphisms. Using the projective packing number, we show that there exists a quantum homomorphism from X to Y if and only if the quantum independence number of a certain product graph achieves |V(X)|. This parallels a well known classical result, and allows us to construct examples of graphs whose independence and quantum independence numbers differ. Most importantly, we show that if there exists a quantum homomorphism from a graph X to a graph Y, then ϑ̄(X) ≤ ϑ̄(Y), where ϑ̄ denotes the Lovász theta function of the complement. We prove similar monotonicity results for projective rank and the projective packing number of the complement, as well as for two variants of ϑ̄. These immediately imply that all of these parameters lie between the quantum clique and quantum chromatic numbers, in particular yielding a quantum analog of the well known ``sandwich theorem". We also briefly investigate the quantum homomorphism order of graphs.

A homomorphism from a graph G to a graph H is a function from V (G) to V (H) that preserves edges. Many combinatorial structures that arise in mathematics and in computer science can be represented naturally as graph homomorphisms and as weighted sums of graph homomorphisms. In this thesis we study the complexity of various problems related to graph homomorphisms.

Homomorfismos de grafos são funções do conjunto de vértices de um grafo no conjunto de vértices de outro grafo que preservam adjacências. O estudo de homomorfismos de grafos é bastante abrangente, existindo muitas linhas de pesquisa sobre esse tópico. Nesta dissertação, apresentaremos resultados sobre homomorfismos de grafos relacionados a pseudo-aleatoriedade, convergência de seqüência de grafos e matrizes de conexão de invariantes de grafos. Esta linha tem se mostrado muito rica, não apenas pelos seus resultados, como também pelas técnicas utilizadas nas demonstrações. Em especial, destacamos a diversidade das ferramentas matemáticas que são usadas, que incluem resultados clássicos de álgebra, probabilidade e análise. / Graph homomorphisms are functions from the vertex set of a graph to the vertex set of another graph that preserve adjacencies. The study of graph homomorphisms is very broad, and there are several lines of research about this topic. In this dissertation, we present results about graph homomorphisms related to convergence of graph sequences and connection matrices of graph parameters. This line of research has been proved to be very rich, not only for its results, but also for the proof techniques. In particular, we highlight the diversity of mathematical tools used, including classical results from Algebra, Probability and Analysis.

Homomorfismos de grafos são funções do conjunto de vértices de um grafo no conjunto de vértices de outro grafo que preservam adjacências. O estudo de homomorfismos de grafos é bastante abrangente, existindo muitas linhas de pesquisa sobre esse tópico. Nesta dissertação, apresentaremos resultados sobre homomorfismos de grafos relacionados a pseudo-aleatoriedade, convergência de seqüência de grafos e matrizes de conexão de invariantes de grafos. Esta linha tem se mostrado muito rica, não apenas pelos seus resultados, como também pelas técnicas utilizadas nas demonstrações. Em especial, destacamos a diversidade das ferramentas matemáticas que são usadas, que incluem resultados clássicos de álgebra, probabilidade e análise. / Graph homomorphisms are functions from the vertex set of a graph to the vertex set of another graph that preserve adjacencies. The study of graph homomorphisms is very broad, and there are several lines of research about this topic. In this dissertation, we present results about graph homomorphisms related to convergence of graph sequences and connection matrices of graph parameters. This line of research has been proved to be very rich, not only for its results, but also for the proof techniques. In particular, we highlight the diversity of mathematical tools used, including classical results from Algebra, Probability and Analysis.

Cette thèse étudie les aspects combinatoires, algorithmiques et la complexité de problèmes de théorie des graphes, et tout spécialement de problèmes liés aux notions de marches, de transitions et de distance dans les graphes. Nous nous intéressons d’abord au problème de traffic monitoring, qui consiste à placer aussi peu de capteurs que possible sur les arcs d’un graphe de façon à pouvoir reconstituer des marches d’objets. La caractérisation d’instances intéressantes dans la pratique nous amène à la notion de transitions interdites, qui renforce le modèle de graphe. Notre travail sur les graphes à transitions interdites comprend aussi l’étude de la notion d’ensemble de transitions connectant, que l’on peut voir comme l’analogue en terme de transitions de la notion d’arbre couvrant. Une partie importante de cette thèse porte sur les graphes géométriques, qui sont des graphes dont les sommets sont des points de l’espace réel et dont les arêtes sont déterminées par les distances géométriques entre les sommets. Ces graphes sont au coeur du célèbre problème de Hadwiger-Nelson et nous sont d’une grande aide dans notre étude de la densité des ensembles qui évitent la distance 1 dans plusieurs types d’espaces normés. Nous développons des outils pour étudier ces problèmes et les utilisons pour prouver la conjecture de Bachoc-Robins sur plusieurs paralléloèdres. Nous nous penchons aussi sur le cas du plan euclidien et améliorons les bornes sur la densité des ensembles évitant la distance 1 et sur son nombre chromatique fractionnaire. Enfin, nous étudions la complexité de problèmes d’homomorphismes de graphes et établissons des théorèmes de dichotomie sur la complexité des homomorphismes localement injectifs vers les tournois réflexifs. / This thesis studies combinatorial, algorithmic and complexity aspects of graph theory problems, and especially of problems related to the notions of walks, transitions and distances in graphs. We first study the problem of traffic monitoring, in which we have to place as few censors as possible on the arcs of a graph to be able to retrace walks of objects. The characterization of instances of practical interests brings us to the notion of forbidden transitions, which strengthens the model of graphs. Our work on forbidden-transition graphs also includes the study of connecting transition sets, which can be seen as a translation to forbidden-transition graphs of the notion of spanning trees. A large part of this thesis focuses on geometric graphs, which are graphs whose vertices are points of the real space and whose edges are determined by geometric distance between the vertices. This graphs are at the core of the famous Hadwiger- Nelson problem and are of great help in our study of the density of sets avoiding distance 1 in various normed spaces. We develop new tools to study these problems and use them to prove the Bachoc-Robins conjecture on several parallelohedra. We also investigate the case of the Euclidean plane and improve the bounds on the density of sets avoiding distance 1 and on its fractional chromatic number. Finally, we study the complexity of graph homomorphism problems and establish dichotomy theorems for the complexity of locally-injective homomorphisms to reflexive tournaments.