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Vertex Colorings without Rainbow Subgraphs

100%
Goddard W., Xu H.
Discussiones Mathematicae Graph Theory
|
2016
|
tom 36
|
nr 4
989-1005
EN
Given a coloring of the vertices of a graph G, we say a subgraph is rainbow if its vertices receive distinct colors. For a graph F, we define the F-upper chromatic number of G as the maximum number of colors that can be used to color the vertices of G such that there is no rainbow copy of F. We present some results on this parameter for certain graph classes. The focus is on the case that F is a star or triangle. For example, we show that the K3-upper chromatic number of any maximal outerplanar graph on n vertices is [n/2] + 1.
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The s-packing chromatic number of a graph

100%
Goddard W., Xu H.
Discussiones Mathematicae Graph Theory
|
2012
|
tom 32
|
nr 4
795-806
EN
Let S = (a₁, a₂, ...) be an infinite nondecreasing sequence of positive integers. An S-packing k-coloring of a graph G is a mapping from V(G) to {1,2,...,k} such that vertices with color i have pairwise distance greater than $a_i$, and the S-packing chromatic number $χ_S(G)$ of G is the smallest integer k such that G has an S-packing k-coloring. This concept generalizes the concept of proper coloring (when S = (1,1,1,...)) and broadcast coloring (when S = (1,2,3,4,...)). In this paper, we consider bounds on the parameter and its relationship with other parameters. We characterize the graphs with $χ_S = 2$ and determine $χ_S$ for several common families of graphs. We examine $χ_S$ for the infinite path and give some exact values and asymptotic bounds. Finally we consider complexity questions, especially about recognizing graphs with $χ_S = 3$.
3
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Worm Colorings

81%
Goddard W., Wash K., Xu H.
Discussiones Mathematicae Graph Theory
|
2015
|
tom 35
|
nr 3
571-584
EN
Given a coloring of the vertices, we say subgraph H is monochromatic if every vertex of H is assigned the same color, and rainbow if no pair of vertices of H are assigned the same color. Given a graph G and a graph F, we define an F-WORM coloring of G as a coloring of the vertices of G without a rainbow or monochromatic subgraph H isomorphic to F. We present some results on this concept especially as regards to the existence, complexity, and optimization within certain graph classes. The focus is on the case that F is the path on three vertices.
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