15 research outputs found
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Όλ¬Έ (λ°μ¬)-- μμΈλνκ΅ λνμ : μ¬λ²λν μνκ΅μ‘κ³Ό, 2018. 2. κΉμλ Ή.The \emph{competition graph} of a digraph is defined to be a graph whose vertex set is the same as and which has an edge joining two distinct vertices and if and only if there are arcs and for some vertex in . Competition graphs have been extensively studied for more than four decades.
Cohen~\cite{cohen1968interval, cohen1977food, cohen1978food} empirically observed that most competition graphs of acyclic digraphs representing food webs are interval graphs. Roberts~\cite{roberts1978food} asked whether or not Cohen's observation was just an artifact of the construction, and then concluded that it was not by showing that if is an arbitrary graph, then together with additional isolated
vertices as many as the number of edges of is the competition graph of some acyclic digraph. Then he asked for a characterization of acyclic digraphs whose competition graphs are interval graphs. Since then, the problem has remained elusive and it has been one of the basic open problems in the study of competition graphs. There have been a lot of efforts to settle the problem and some progress has been made. While Cho and Kim~\cite{cho2005class} tried to answer his question, they could show that the competition graphs of doubly partial orders are interval graphs. They also showed that an interval graph together with sufficiently many isolated vertices is the competition graph of a doubly partial order.
In this thesis, we study the competition graphs of -partial orders some of which generalize the results on the competition graphs of doubly partial orders.
For a positive integer , a digraph is called a \emph{-partial order} if V(D) \subset \RR^d and there is an arc from a vertex to a vertex if and only if is componentwise greater than . A doubly partial order is a -partial order.
We show that every graph is the competition graph of a -partial order for some nonnegative integer , call the smallest such the \emph{partial order competition dimension} of , and denote it by .
This notion extends the statement that the competition graph of a doubly partial order is interval and the statement that any interval graph can be the competition graph of a doubly partial order as long as sufficiently many isolated vertices are added, which were proven by Cho and Kim~\cite{cho2005class}. Then we study the partial order competition dimensions of some interesting families of graphs. We also study the -step competition graphs and the competition hypergraph of -partial orders.1 Introduction 1
1.1 Basic notions in graph theory 1
1.2 Competition graphs 6
1.2.1 A brief history of competition graphs 6
1.2.2 Competition numbers 7
1.2.3 Interval competition graphs 10
1.3 Variants of competition graphs 14
1.3.1 m-step competition graphs 15
1.3.2 Competition hypergraphs 16
1.4 A preview of the thesis 18
2 On the competition graphs of d-partial orders 1 20
2.1 The notion of d-partial order 20
2.2 The competition graphs of d-partial orders 21
2.2.1 The regular (d β 1)-dimensional simplex β³ dβ1 (p) 22
2.2.2 A bijection from H d + to a set of regular (d β 1)-simplices 23
2.2.3 A characterization of the competition graphs of d-partial orders 25
2.2.4 Intersection graphs and competition graphs of d-partial orders 27
2.3 The partial order competition dimension of a graph 29
3 On the partial order competition dimensions of chordal graphs 2 38
3.1 Basic properties on the competition graphs of 3-partial orders 39
3.2 The partial order competition dimensions of diamond-free chordal graphs 42
3.3 Chordal graphs having partial order competition dimension greater than three 46
4 The partial order competition dimensions of bipartite graphs 3 53
4.1 Order types of two points in R 3 53
4.2 An upper bound for the the partial order competition dimension of a graph 57
4.3 Partial order competition dimensions of bipartite graphs 64
5 On the m-step competition graphs of d-partial orders 4 69
5.1 A characterization of the m-step competition graphs of dpartial orders 69
5.2 Partial order m-step competition dimensions of graphs 71
5.3 dim poc (Gm) in the aspect of dim poc (G) 76
5.4 Partial order competition exponents of graphs 79
6 On the competition hypergraphs of d-partial orders 5 81
6.1 A characterization of the competition hypergraphs of d-partial orders 81
6.2 The partial order competition hyper-dimension of a hypergraph 82
6.3 Interval competition hypergraphs 88
Abstract (in Korean) 99Docto
Edge Clique Cover of Claw-free Graphs
The smallest number of cliques, covering all edges of a graph , is
called the (edge) clique cover number of and is denoted by . It
is an easy observation that for every line graph with vertices,
. G. Chen et al. [Discrete Math. 219 (2000), no. 1--3, 17--26;
MR1761707] extended this observation to all quasi-line graphs and questioned if
the same assertion holds for all claw-free graphs. In this paper, using the
celebrated structure theorem of claw-free graphs due to Chudnovsky and Seymour,
we give an affirmative answer to this question for all claw-free graphs with
independence number at least three. In particular, we prove that if is a
connected claw-free graph on vertices with , then and equality holds if and only if is either the graph of
icosahedron, or the complement of a graph on vertices called twister or
the power of the cycle , for .Comment: 74 pages, 4 figure