Dynamic coloring of graphs

In this dissertation, we introduce and study the idea of a dynamic coloring of a graph, a coloring in which any multiple-degree vertex of the graph must be adjacent to at least two color classes.;As parts of the overall research, we study (for some interesting subjects of colorings) the corresponding subjects of dynamic colorings, we compare the chromatic number and dynamic chromatic number, and we study some problems unique to dynamic colorings. Also, we introduce and briefly study a generalization of dynamic coloring.;The interesting subjects of colorings we consider are the chromatic number of important graphs, upper bounds of the chromatic number, vertex-critical graphs, and stable graphs. For these first three subjects, we prove theorems for dynamic colorings that are similar to important theorems known for proper colorings, while we show no such theorems exist for stable graphs.;We make an extensive comparison of the two chromatic numbers that includes a description of graphs for which the two chromatic numbers are equal, that presents a class of graphs for which the per-graph differences in the two chromatic numbers is unbounded, that shows the difference is at most two for any K1,3-free graph, and that studies the difference for regular graphs.;In our study of some unique problems of dynamic colorings, we characterize the graphs for which the dynamic chromatic number equals the number of vertices, we characterize the graphs for which the dynamic chromatic number equals one less than the number of vertices, we characterize the graphs for which the deletion of some vertex causes the dynamic chromatic number to decrease by more than one, and we obtain strong results describing graphs for which the removal of any vertex causes the dynamic chromatic number to increase.;Finally, we introduce and briefly study a generalization of dynamic coloring

b-coloring is NP-hard on co-bipartite graphs and polytime solvable on tree-cographs

A b-coloring of a graph is a proper coloring such that every color class contains a vertex that is adjacent to all other color classes. The b-chromatic number of a graph G, denoted by \chi_b(G), is the maximum number t such that G admits a b-coloring with t colors. A graph G is called b-continuous if it admits a b-coloring with t colors, for every t = \chi(G),\ldots,\chi_b(G), and b-monotonic if \chi_b(H_1) \geq \chi_b(H_2) for every induced subgraph H_1 of G, and every induced subgraph H_2 of H_1. We investigate the b-chromatic number of graphs with stability number two. These are exactly the complements of triangle-free graphs, thus including all complements of bipartite graphs. The main results of this work are the following: - We characterize the b-colorings of a graph with stability number two in terms of matchings with no augmenting paths of length one or three. We derive that graphs with stability number two are b-continuous and b-monotonic. - We prove that it is NP-complete to decide whether the b-chromatic number of co-bipartite graph is at most a given threshold. - We describe a polynomial time dynamic programming algorithm to compute the b-chromatic number of co-trees. - Extending several previous results, we show that there is a polynomial time dynamic programming algorithm for computing the b-chromatic number of tree-cographs. Moreover, we show that tree-cographs are b-continuous and b-monotonic

Coloring, List Coloring, and Painting Squares of Graphs (and other related problems)

We survey work on coloring, list coloring, and painting squares of graphs; in particular, we consider strong edge-coloring. We focus primarily on planar graphs and other sparse classes of graphs.Comment: 32 pages, 13 figures and tables, plus 195-entry bibliography, comments are welcome, published as a Dynamic Survey in Electronic Journal of Combinatoric