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3-transitive digraphs

César Hernández-Cruz (2012)

Discussiones Mathematicae Graph Theory

Let D be a digraph, V(D) and A(D) will denote the sets of vertices and arcs of D, respectively. A digraph D is 3-transitive if the existence of the directed path (u,v,w,x) of length 3 in D implies the existence of the arc (u,x) ∈ A(D). In this article strong 3-transitive digraphs are characterized and the structure of non-strong 3-transitive digraphs is described. The results are used, e.g., to characterize 3-transitive digraphs that are transitive and to characterize 3-transitive digraphs with...

4-cycle properties for characterizing rectagraphs and hypercubes

Khadra Bouanane, Abdelhafid Berrachedi (2017)

Czechoslovak Mathematical Journal

A ( 0 , 2 ) -graph is a connected graph, where each pair of vertices has either 0 or 2 common neighbours. These graphs constitute a subclass of ( 0 , λ ) -graphs introduced by Mulder in 1979. A rectagraph, well known in diagram geometry, is a triangle-free ( 0 , 2 ) -graph. ( 0 , 2 ) -graphs include hypercubes, folded cube graphs and some particular graphs such as icosahedral graph, Shrikhande graph, Klein graph, Gewirtz graph, etc. In this paper, we give some local properties of 4-cycles in ( 0 , λ ) -graphs and more specifically in ( 0 , 2 ) -graphs,...

4-Transitive Digraphs I: The Structure of Strong 4-Transitive Digraphs

César Hernández-Cruz (2013)

Discussiones Mathematicae Graph Theory

Let D be a digraph, V (D) and A(D) will denote the sets of vertices and arcs of D, respectively. A digraph D is transitive if for every three distinct vertices u, v,w ∈ V (D), (u, v), (v,w) ∈ A(D) implies that (u,w) ∈ A(D). This concept can be generalized as follows: A digraph is k-transitive if for every u, v ∈ V (D), the existence of a uv-directed path of length k in D implies that (u, v) ∈ A(D). A very useful structural characterization of transitive digraphs has been known for a long time, and...

5-Stars of Low Weight in Normal Plane Maps with Minimum Degree 5

Oleg V. Borodin, Anna O. Ivanova, Tommy R. Jensen (2014)

Discussiones Mathematicae Graph Theory

It is known that there are normal plane maps M5 with minimum degree 5 such that the minimum degree-sum w(S5) of 5-stars at 5-vertices is arbitrarily large. In 1940, Lebesgue showed that if an M5 has no 4-stars of cyclic type (5, 6, 6, 5) centered at 5-vertices, then w(S5) ≤ 68. We improve this bound of 68 to 55 and give a construction of a (5, 6, 6, 5)-free M5 with w(S5) = 48

A backward selection procedure for approximating a discrete probability distribution by decomposable models

Francesco M. Malvestuto (2012)

Kybernetika

Decomposable (probabilistic) models are log-linear models generated by acyclic hypergraphs, and a number of nice properties enjoyed by them are known. In many applications the following selection problem naturally arises: given a probability distribution p over a finite set V of n discrete variables and a positive integer k , find a decomposable model with tree-width k that best fits p . If ℋ is the generating hypergraph of a decomposable model and p ℋ is the estimate of p under the model, we can measure...

A bi-average tree solution for probabilistic communication situations with fuzzy coalition

Xianghui Li, Hao Sun, Dongshuang Hou (2019)

Kybernetika

A probabilistic communication structure considers the setting with communication restrictions in which each pair of players has a probability to communicate directly. In this paper, we consider a more general framework, called a probabilistic communication structure with fuzzy coalition, that allows any player to have a participation degree to cooperate within a coalition. A maximal product spanning tree, indicating a way of the greatest possibility to communicate among the players, is introduced...

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