Note on graphs colouring

Dănuţ Marcu

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On integral sum graphs with a saturated vertex

Zhibo Chen (2010)

Czechoslovak Mathematical Journal

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As introduced by F. Harary in 1994, a graph $G$ is said to be an $i n t e g r a l$ $s u m$ $g r a p h$ if its vertices can be given a labeling $f$ with distinct integers so that for any two distinct vertices $u$ and $v$ of $G$ , $u v$ is an edge of $G$ if and only if $f (u) + f (v) = f (w)$ for some vertex $w$ in $G$ . We prove that every integral sum graph with a saturated vertex, except the complete graph $K_{3}$ , has edge-chromatic number equal to its maximum degree. (A vertex of a graph $G$ is said to be if it is adjacent to every...

Radio number for some thorn graphs

Ruxandra Marinescu-Ghemeci (2010)

Discussiones Mathematicae Graph Theory

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For a graph G and any two vertices u and v in G, let d(u,v) denote the distance between u and v and let diam(G) be the diameter of G. A multilevel distance labeling (or radio labeling) for G is a function f that assigns to each vertex of G a positive integer such that for any two distinct vertices u and v, d(u,v) + |f(u) - f(v)| ≥ diam(G) + 1. The largest integer in the range of f is called the span of f and is denoted span(f). The radio number of G, denoted rn(G), is the minimum span...

Homogeneously embedding stratified graphs in stratified graphs

Gary Chartrand, Donald W. Vanderjagt, Ping Zhang (2005)

Mathematica Bohemica

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A 2-stratified graph $G$ is a graph whose vertex set has been partitioned into two subsets, called the strata or color classes of $G$ . Two $2$ -stratified graphs $G$ and $H$ are isomorphic if there exists a color-preserving isomorphism $φ$ from $G$ to $H$ . A $2$ -stratified graph $G$ is said to be homogeneously embedded in a $2$ -stratified graph $H$ if for every vertex $x$ of $G$ and every vertex $y$ of $H$ , where $x$ and $y$ are colored the same, there exists an induced $2$ -stratified subgraph $H^{'}$ of $H$ containing $y$ and a color-preserving...

Minus total domination in graphs

Hua Ming Xing, Hai-Long Liu (2009)

Czechoslovak Mathematical Journal

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A three-valued function $f V \to {- 1, 0, 1}$ defined on the vertices of a graph $G = (V, E)$ is a minus total dominating function (MTDF) if the sum of its function values over any open neighborhood is at least one. That is, for every $v \in V$ , $f (N (v)) \geq 1$ , where $N (v)$ consists of every vertex adjacent to $v$ . The weight of an MTDF is $f (V) = \sum f (v)$ , over all vertices $v \in V$ . The minus total domination number of a graph $G$ , denoted $γ_{t}^{-} (G)$ , equals the minimum weight of an MTDF of $G$ . In this paper, we discuss some properties of minus total domination on a graph...

Vertex-disjoint stars in graphs

Katsuhiro Ota (2001)

Discussiones Mathematicae Graph Theory

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In this paper, we give a sufficient condition for a graph to contain vertex-disjoint stars of a given size. It is proved that if the minimum degree of the graph is at least k+t-1 and the order is at least (t+1)k + O(t²), then the graph contains k vertex-disjoint copies of a star $K_{1, t}$ . The condition on the minimum degree is sharp, and there is an example showing that the term O(t²) for the number of uncovered vertices is necessary in a sense.

Bounds concerning the alliance number

Grady Bullington, Linda Eroh, Steven J. Winters (2009)

Mathematica Bohemica

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P. Kristiansen, S. M. Hedetniemi, and S. T. Hedetniemi, in Alliances in graphs, J. Combin. Math. Combin. Comput. 48 (2004), 157–177, and T. W. Haynes, S. T. Hedetniemi, and M. A. Henning, in Global defensive alliances in graphs, Electron. J. Combin. 10 (2003), introduced the defensive alliance number $a (G)$ , strong defensive alliance number $\hat{a} (G)$ , and global defensive alliance number $γ_{a} (G)$ . In this paper, we consider relationships between these parameters and the domination number $γ (G)$ . For any positive...