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1
Content available remote

Products Of Digraphs And Their Competition Graphs

100%
Sonntag M., Teichert H.-M.
Discussiones Mathematicae Graph Theory
|
2016
|
tom 36
|
nr 1
43-58
EN
If D = (V, A) is a digraph, its competition graph (with loops) CGl(D) has the vertex set V and {u, v} ⊆ V is an edge of CGl(D) if and only if there is a vertex w ∈ V such that (u, w), (v, w) ∈ A. In CGl(D), loops {v} are allowed only if v is the only predecessor of a certain vertex w ∈ V. For several products D1 ⚬ D2 of digraphs D1 and D2, we investigate the relations between the competition graphs of the factors D1, D2 and the competition graph of their product D1 ⚬ D2.
2
Content available remote

The niche graphs of interval orders

80%
Park J., Sano Y.
Discussiones Mathematicae Graph Theory
|
2014
|
tom 34
|
nr 2
353-359
EN
The niche graph of a digraph D is the (simple undirected) graph which has the same vertex set as D and has an edge between two distinct vertices x and y if and only if N+D(x) ∩ N+D(y) ≠ ∅ or N−D(x) ∩ N−D(y) ≠ ∅, where N+D(x) (resp. N−D(x)) is the set of out-neighbors (resp. in-neighbors) of x in D. A digraph D = (V,A) is called a semiorder (or a unit interval order ) if there exist a real-valued function f : V → R on the set V and a positive real number δ ∈ R such that (x, y) ∈ A if and only if f(x) > f(y)+δ. A digraph D = (V,A) is called an interval order if there exists an assignment J of a closed real interval J(x) ⊂ R to each vertex x ∈ V such that (x, y) ∈ A if and only if min J(x) > max J(y). Kim and Roberts characterized the competition graphs of semiorders and interval orders in 2002, and Sano characterized the competition-common enemy graphs of semiorders and interval orders in 2010. In this note, we give characterizations of the niche graphs of semiorders and interval orders
3
Content available remote

The Phylogeny Graphs of Doubly Partial Orders

80%
Park B., Sano Y.
Discussiones Mathematicae Graph Theory
|
2013
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tom 33
|
nr 4
657-664
EN
The competition graph of a doubly partial order is known to be an interval graph. The CCE graph and the niche graph of a doubly partial order are also known to be interval graphs if the graphs do not contain a cycle of length four and three as an induced subgraph, respectively. Phylogeny graphs are variant of competition graphs. The phylogeny graph P(D) of a digraph D is the (simple undirected) graph defined by V (P(D)) := V (D) and E(P(D)) := {xy | N+D (x) ∩ N+D(y) ¹ ⊘ } ⋃ {xy | (x,y) ∈ A(D)}, where N+D(x):= {v ∈ V(D) | (x,v) ∈ A (D)}. In this note, we show that the phylogeny graph of a doubly partial order is an interval graph. We also show that, for any interval graph G̃, there exists an interval graph G such that G̃ contains the graph G as an induced subgraph and that G̃ is the phylogeny graph of a doubly partial order.
4
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Competition hypergraphs of digraphs with certain properties II. Hamiltonicity

80%
Sonntag M., Teichert H.-M.
Discussiones Mathematicae Graph Theory
|
2008
|
tom 28
|
nr 1
23-34
EN
If D = (V,A) is a digraph, its competition hypergraph 𝓒𝓗 (D) has vertex set V and e ⊆ V is an edge of 𝓒𝓗 (D) iff |e| ≥ 2 and there is a vertex v ∈ V, such that $e = N_D⁻(v) = {w ∈ V|(w,v) ∈ A}$. We give characterizations of 𝓒𝓗 (D) in case of hamiltonian digraphs D and, more general, of digraphs D having a τ-cycle factor. The results are closely related to the corresponding investigations for competition graphs in Fraughnaugh et al. [4] and Guichard [6].
5
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Competition hypergraphs of digraphs with certain properties I. Strong connectedness

80%
Sonntag M., Teichert H.-M.
Discussiones Mathematicae Graph Theory
|
2008
|
tom 28
|
nr 1
5-21
EN
If D = (V,A) is a digraph, its competition hypergraph 𝓒𝓗(D) has the vertex set V and e ⊆ V is an edge of 𝓒𝓗(D) iff |e| ≥ 2 and there is a vertex v ∈ V, such that e = {w ∈ V|(w,v) ∈ A}. We tackle the problem to minimize the number of strong components in D without changing the competition hypergraph 𝓒𝓗(D). The results are closely related to the corresponding investigations for competition graphs in Fraughnaugh et al. [3].
6
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The competition numbers of Johnson graphs

80%
Kim S.-R., Park B., Sano Y.
Discussiones Mathematicae Graph Theory
|
2010
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tom 30
|
nr 3
449-459
EN
The competition graph of a digraph D is a graph which has the same vertex set as D and has an edge between two distinct vertices x and y if and only if there exists a vertex v in D such that (x,v) and (y,v) are arcs of D. For any graph G, G together with sufficiently many isolated vertices is the competition graph of some acyclic digraph. The competition number k(G) of a graph G is defined to be the smallest number of such isolated vertices. In general, it is hard to compute the competition number k(G) for a graph G and to characterize all graphs with given competition number k has been one of the important research problems in the study of competition graphs. The Johnson graph J(n,d) has the vertex set ${v_X | X ∈ \binom{[n]}{d}$, where $\binom{[n]}{d}$ denotes the set of all d-subsets of an n-set [n] = {1,..., n}, and two vertices $v_{X₁}$ and $v_{X₂}$ are adjacent if and only if |X₁ ∩ X₂| = d - 1. In this paper, we study the edge clique number and the competition number of J(n,d). Especially we give the exact competition numbers of J(n,2) and J(n,3).
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