Resumen de Colored combinatorial structures: homomorphisms and counting

Amanda Montejano Cantoral

• Starting with the four color problem, the theory of graph coloring has existed for more than 150 years, It deals with the fundamental problem of partitioning a set of objects into classes according to certain rules. From this modest beginning, the theory has become central in discrete mathematics, with many contemporary generalizations and applications. In this thesis, our particular interest is in two very active areas of research which have emerged from coloring problems: Graph Homomorphism Theory and Arithmetic Ramsey Theory.

  Graph Homomorphism Theory can be described as the study of classes of combinatorial structures under natural morphisms. The chromatic number of a simple graph G can be stated, in this context, as the smallest complete graph to which G admits a homomorphism. Thus Graph Homomorphism Theory has been extensively studied as a generalization of colorings. An excellent reference in the subject is the book by Hell and Nesetril {Graphs and homomorphisms, Oxf. Univ. Press, 2004}.

  Ramsey Theory studies the existence of particular color patterns in colored structures. Starting with the Theorems of Ramsey, Hilbert, Schur and van der Waerden, the theory has developed as a wide and beautiful area of combinatorics, in which a great variety of techniques are used from many branches of mathematics.

  Many of the classical results in the area are arithmetic versions of the theory and we are interested in this particular branch of Ramsey Theory. A good reference in the area is the book of Langman and Robertson {Ramsey Theory on the Integers, Stud. Math. Lib. 24, AMS, 2003}.

  This thesis is organized in two parts. The first part deals with the study of homomorphisms in the class of colored mixed graphs, which are graphs with vertices linked by both colored arcs and colored edges. The chromatic number of such a graph G is defined as the smallest order of a colored mixed graph H such that there exists a (color preserving) homomorphism from G to H. These notions were introduced by Nesetril and Raspaud in {Colored homomorphisms of colored mixed graphs, J. C. T. Ser. B 80 (2000)}. Generalizing known results for the class of oriented graphs we study the colored mixed chromatic number of paths, trees, graphs with bounded acyclic chromatic number, graphs of bounded treewidth, planar graphs, outerplanar graphs and sparse graphs. In particular we give the exact chormatic number of planar graphs and of partial 2-trees with appropriately large girth. Motivated by the dichotomy conjecture for relational structures we focuss on the class of 2-edge colored graphs and study its relationship with the class of oriented graphs. In particular we consider the characterization of cores and of duality pairs in this class.

  The second part of the thesis is related to Arithmetic Ramsey Theory. We consider the existence and the enumeration of colored structures, mainly monochromatic or rainbow structures, in colorings of finite groups.

  The structures under consideration can be described as solutions of systems of equations in the group, the main examples being arithmetic progressions and Schur triples. We give a structural description of those colorings in abelian groups which do not contain 3-term arithmetic progressions with its members having pairwise distinct colors. This structural description proves a conjecture of Jungic et al. {Rainbow Ramsey Theory. Integers: E. J. C. N. T. 5(2) A9. (2005)} on the size of the smallest chromatic class of such colorings in cyclic groups.