A digraph obtained by replacing each edge of a complete multipartite graph by an arc or a pair of mutually opposite arcs with the same end vertices is called a semicomplete multipartite digraph. L. Volkmann (1998) raised the following question: Let D be a strong semicomplete multipartite digraph with a longest path of length l. Does there exist a strong spanning oriented subgraph of D with a lo...

A consecutive-d digraph is a digraph G(d, n, q, T) whose n nodes are labeled by the residues modulo n and a link from node i to node j exists if and only if j I pi + k (mod n) for some k with r 5 k 5 r + d 1. Consecutive-d digraphs are used as models for many computer networks and multiprocessor systems, in which the existence of a Hamiltonian circuit is important. Conditions for a consecutive-...

Let G = (V,E) be an undirected graph and let π = {V1, V2, . . . , Vk} be a partition of the vertices V of G into k blocks Vi. From this partition one can construct the following digraph D(π) = (π,E(π)), the vertices of which correspond one-to-one with the k blocks Vi of π, and there is an arc from Vi to Vj if every vertex in Vj is adjacent to at least one vertex in Vi, that is, Vi dominates Vj ...

A digraph D is primitive if, for some positive integer r, there is a u ! v walk of length r for every pair u; v of vertices of D. The minimum such r is called the exponent of D, denoted exp(D). We prove that if a primitive digraph D of order n is an Abelian Cayley digraph with degree k 4, then exp(D) max n 5; n dk=2e 1 o : If we assume only that the primitive digraph D is vertex-transitive, the...

In a directed graph, a star is an arborescence with at least one arc, in which the root dominates all the other vertices. A galaxy is a vertex-disjoint union of stars. In this paper, we consider the Spanning Galaxy Problem of deciding whether a digraph D has a spanning galaxy or not. We show that although this problem is NP-complete (even when restricted to acyclic digraphs), it becomes polynom...

Let D be a digraph. Let k be a natural number with k 2. A set J ⊆ V (D) will be called a k-kernel of the digraph D iff: 1) For each x, x′ ∈ J , x = x′ we have dD(x, x′) k and 2) For each y ∈ V (D) − J , there exists x ∈ J such that dD(y, x) ≤ k − 1. In [4], for any digraph D we constructed a digraph s(S) such that D has a k-kernel iff s(S) has a k-kernel. In this paper we prove that the number ...

The eccentricity e(v) of a vertex v in a strongly connected digraph G is the maximum distance from v. The eccentricity sequence of a digraph is the list of eccentricities of its vertices given in nondecreasing order. A sequence of positive integers is a digraphical eccentric sequence if it is the eccentricity sequence of some digraph. A set of positive integers S is a digraphical eccentric set ...

To every graph (or digraph) A, there is an associated automorphism group Aut(A). Frucht’s theorem asserts the converse association; that for any finite group G there is a graph (or digraph) A for which Aut(A) ∼= G. A new operation on digraphs was introduced recently as an aid in solving certain questions regarding cancellation over the direct product of digraphs. Given a digraph A, its factoria...

In the first part of this paper we give necessary and sufficient conditions for some special classes of digraphs to have a (k, l)-kernel. One of them is the duplication of a set of vertices in a digraph. This duplication come into being as the generalization of the duplication of a vertex in a graph (see [4]). Another one is the D-join of a digraph D and a sequence α of nonempty pairwise disjoi...

A digraph D is k-transitive if the existence of a directed path (v0, v1, . . . , vk), of length k implies that (v0, vk) ∈ A(D). Clearly, a 2-transitive digraph is a transitive digraph in the usual sense. Transitive digraphs have been characterized as compositions of complete digraphs on an acyclic transitive digraph. Also, strong 3 and 4-transitive digraphs have been characterized. In this work...