Greene's Theorem states that the maximum cardinality of an optimal k-path in a poset is equal to the minimum k-norm of a k-optimal coloring. This result was extended to all acyclic digraphs, and is conjectured to hold for general digraphs. We prove the result for general digraphs in which an optimal k-path contains a path of cardinality one. This implies the validity of the conjecture for all b...

The dichromatic number ~ χ(D) of a digraph D is the least number k such that the vertex set of D can be partitioned into k parts each of which induces an acyclic subdigraph. Introduced by Neumann-Lara in 1982, this digraph invariant shares many properties with the usual chromatic number of graphs and can be seen as the natural analog of the graph chromatic number. In this paper, we study the li...

For given two digraphs, we can construct a larger digraph through join. The digraphs that make up the join are called factors of In this paper, give necessary and sufficient condition function on determined by discrete Morse functions is function. Moreover, further prove theory when satisfy certain conditions.

A graph $G$ is called $P_4$-free, if $G$ does not contain an induced subgraph $P_4$. The domination polynomial of a graph $G$ of order $n$ is the polynomial $D(G,x)=sum_{i=1}^{n} d(G,i) x^{i}$, where $d(G,i)$ is the number of dominating sets of $G$ of size $i$. Every root of $D(G,x)$ is called a domination root of $G$. In this paper we state and prove formula for the domination polynomial of no...

In this paper, we study the kth upper bases of primitive non-powerful signed digraphs. A bound on the kth upper bases of all primitive non-powerful signed digraphs is obtained, and the equality case of the bound is characterized. We also show that there exists “gap” in the kth upper base set of primitive non-powerful signed digraphs. AMS classification: 05C20, 05C50, 15A48

The Klee-Quaife problem is finding the minimum order μ(d, c, v) of the (d, c, v) graph, which is a c-vertex connected v-regular graph with diameter d. Many authors contributed finding μ(d, c, v) and they also enumerated and classified the graphs in several cases. This problem is naturally extended to the case of digraphs. So we are interested in the extended Klee-Quaife problem. In this paper, ...

We derive the asymptotic distribution of the domination number of a new family of random digraph called proximity catch digraph (PCD), which has application to statistical testing of spatial point patterns and to pattern recognition. The PCD we use is a parametrized digraph based on two sets of points on the plane, where sample size and locations of the elements of one is held fixed, while the ...

In this paper, a colouring game and two versions of marking games (the weak and the strong) on digraphs are studied. We introduce the weak game chromatic number χwg(D) and the weak game colouring number wgcol(D) of digraphs D. It is proved that if D is an oriented planar graph, then χwg(D) 6 wgcol(D) 6 9, and if D is an oriented outerplanar graph, then χwg(D) 6 wgcol(D) 6 4. Then we study the s...

twin reversed arterial perfusion (trap) sequence is a syndrome with poor prognosis, seen only in monochorionic monozygotic twin pregnancies. the incidence is one in 35.000 births and one in 100 monozygotic twin pregnancies. it is characterized with a recipient fetus exhibiting lethal anomalies including acardia and a pump fetus. mortality is usually due to heart failure or premature labor cause...