Containment and overlap representations of digraphs are studied, with the following results. The interval containment digraphs are the digraphs of Ferrers dimension 2, and the circular-arc containment digraphs are the complements of circular-arc intersection digraphs. A poset is an interval containment poset if and only if its comparability digraph is an interval containment digraph, and a grap...

In this paper, we study two types of strong subgraph packing problems in digraphs, including internally disjoint problem and arc-disjoint problem. These can be viewed as generalizations the famous Steiner tree are closely related to arc decomposition We first prove NP-completeness for restricted symmetric digraphs Eulerian digraphs. Then get inapproximability results Finally digraph composition...

We introduce the price of symmetrisation, a concept that aims to compare fundamental differences (gap and quotient) between values of a given graph invariant for digraphs and the values of the same invariant of the symmetric versions of these digraphs. Basically, given some invariant our goal is to characterise digraphs that maximise price of symmetrisation. In particular, we show that for some...

A digraph D = (V,E) is a mod difference digraph if there exist a positive integer m and a labeling L : V → {1, 2, . . . , m − 1} such that (x, y) ∈ E if and only if L(y) − L(x) ≡ L(w)(mod m) for some w ∈ V. In this paper we prove that complete symmetric digraphs, unipaths and unicycles are mod difference digraphs.

We are interested in the construction of the largest possible vertex symmetric digraphs with the property that between any two vertices there is a walk of length two (that is, they are 2-reachable). Other than a theoretical interest, these digraphs and their generalizations may be used as models of interconnection networks for implementing parallelism. In these systems many nodes are connected ...

We show that an action of SL(2, p), p ≥ 7 an odd prime such that 4 6 | (p − 1), has exactly two orbital digraphs Γ1, Γ2, such that Aut(Γi) admits a complete block system B of p + 1 blocks of size 2, i = 1, 2, with the following properties: the action of Aut(Γi) on the blocks of B is nonsolvable, doubly-transitive, but not a symmetric group, and the subgroup of Aut(Γi) that fixes each block of B...