c-Pancyclic Partial Ordering and （c-1）-Pan-Outpath Partial Ordering in Semicomplete Multipartite Digraphs

An outpath of a vertex v in a digraph is a path starting at v such that v dominates the end vertex of the path only if the end vertex also dominates v.First we show that letting D be a strongly connected semicomplete c-partite digraph (c≥3)1 and one of the partite sets of it consists of a single vertex, say v, then D has a c-pancyclic partial ordering from v, which generalizes a result about pancyclicity of multipartite tournaments obtained by Gutin in 1993.Then we prove that letting D be a strongly connected semicomplete c-partite digraph with c≥3 and letting v be a vertex of D,then Dhas a(c-1)-pan-outpath partly ordering from v.This result improves a theorem about outpaths in semicomplete multipartite digraphs obtained by Guo in 1999.     

Locally semicomplete digraphs: A generalization of tournaments

In this paper we introduce a new class of directed graphs called locally semicomplete digraphs. These are defined to be those digraphs for which the following holds: for every vertex x the vertices dominated by x induce a semicomplete digraph and the vertices that dominate x induce a semicomplete digraph. (A digraph is semicomplete if for any two distinct vertices u and ν, there is at least one arc between them.) This class contains the class of semicomplete digraphs, but is much more general. In fact, the class of underlying graphs of the locally semi-complete digraphs is precisely the class of proper circular-arc graphs (see [13], Theorem 3). We show that many of the classic theorems for tournaments have natural analogues for locally semicomplete digraphs. For example, every locally semicomplete digraph has a directed Hamiltonian path and every strong locally semicomplete digraph has a Hamiltonian cycle. We also consider connectivity properties, domination orientability, and algorithmic aspects of locally semicomplete digraphs. Some of the results on connectivity are new, even when restricted to semicomplete digraphs.     

Kings in semicomplete multipartite digraphs

A digraph obtained by replacing each edge of a complete p‐partite graph by an arc or a pair of mutually opposite arcs with the same end vertices is called a semicomplete p‐partite digraph, or just a semicomplete multipartite digraph. A semicomplete multipartite digraph with no cycle of length two is a multipartite tournament. In a digraph D, an r‐king is a vertex q such that every vertex in D can be reached from q by a path of length at most r. Strengthening a theorem by K. M. Koh and B. P. Tan (Discr Math 147 (1995), 171–183) on the number of 4‐kings in multipartite tournaments, we characterize semicomplete multipartite digraphs, which have exactly k 4‐kings for every k = 1, 2, 3, 4, 5. © 2000 John Wiley & Sons, Inc. J Graph Theory 33: 177‐183, 2000     

ON THE EXPONENT SET OF PRIMITIVELOCALLY SEMICOMPLETE DIGRAPHS

A locally semicomplete digraph is a digraph D=(V,A) satisfying the following condi-tion for every vertex x∈V the D[O(x)] and D[I(x)] are semicomplete digraphs. In this paper,we get some properties of cycles and determine the exponent set of primitive locally semicompleted digraphs.     

A Characterization of Locally Semicomplete CKI-digraphs

A digraph is locally-in semicomplete if for every vertex of D its in-neighborhood induces a semicomplete digraph and it is locally semicomplete if for every vertex of D the in-neighborhood and the out-neighborhood induces a semicomplete digraph. The locally semicomplete digraphs where characterized in 1997 by Bang-Jensen et al. and in 1998 Bang-Jensen and Gutin posed the problem if finding a kernel in a locally-in semicomplete digraph is polynomial or not. A kernel of a digraph is a set of vertices, which is independent and absorbent. A digraph D such that every proper induced subdigraph of D has a kernel is said to be critical kernel imperfect digraph (CKI-digraph) if the digraph D does not have a kernel. A digraph without an induced CKI-digraph as a subdigraph does have a kernel. We characterize the locally semicomplete digraphs, which are CKI. As a consequence of this characterization we conclude that determinate whether a locally semicomplete digraph is a CKI-digraph or not, is polynomial.     

Solution of a Conjecture of Volkmann on the Number of Vertices in Longest Paths and Cycles of Strong Semicomplete Multipartite Digraphs

 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 conjectured that l≤2c−1, where l (c, respectively) is the number of vertices in a longest path (longest cycle) of a strong semicomplete multipartite digraph. The bound on l is sharp. We settle this conjecture in affirmative. Received: October 26, 1998?Final version received: August 16, 1999     

Kings in locally semicomplete digraphs

A k‐king in a digraph D is a vertex which can reach every other vertex by a directed path of length at most k. We consider k‐kings in locally semicomplete digraphs and mainly prove that all strong locally semicomplete digraphs which are not round decomposable contain a 2‐king. © 2009 Wiley Periodicals, Inc. J Graph Theory 63: 279–287, 2010     

Spanning 2-strong tournaments in 3-strong semicomplete digraphs

We prove that every 3-strong semicomplete digraph on at least 5 vertices contains a spanning 2-strong tournament. Our proof is constructive and implies a polynomial algorithm for finding a spanning 2-strong tournament in a given 3-strong semicomplete digraph. We also show that there are infinitely many (2k−2)-strong semicomplete digraphs which contain no spanning k-strong tournament and conjecture that every(2k−1)-strong semicomplete digraph which is not the complete digraph on 2k vertices contains a spanning k-strong tournament.     

Strongly quasi-Hamiltonian-connected semicomplete multipartite digraphs

A semicomplete multipartite or semicomplete c$c$-partite digraph D$D$ is a biorientation of a c$c$-partite graph. A semicomplete multipartite digraph D$D$ is called strongly quasi-Hamiltonian-connected, if for any two distinct vertices x$x$ and y$y$ of D$D$, there is a path P$P$ from x$x$ to y$y$ such that P$P$ contains at least one vertex from each partite set of D$D$.     

Disjoint quasi‐kernels in digraphs

A quasi‐kernel in a digraph is an independent set of vertices such that any vertex in the digraph can reach some vertex in the set via a directed path of length at most two. Chvátal and Lovász proved that every digraph has a quasi‐kernel. Recently, Gutin et al. raised the question of which digraphs have a pair of disjoint quasi‐kernels. Clearly, a digraph has a pair of disjoint quasi‐kernels cannot contain sinks, that is, vertices of outdegree zero, as each such vertex is necessarily included in a quasi‐kernel. However, there exist digraphs which contain neither sinks nor a pair of disjoint quasi‐kernels. Thus, containing no sinks is not sufficient in general for a digraph to have a pair of disjoint quasi‐kernels. In contrast, we prove that, for several classes of digraphs, the condition of containing no sinks guarantees the existence of a pair of disjoint quasi‐kernels. The classes contain semicomplete multipartite, quasi‐transitive, and locally semicomplete digraphs. © 2008 Wiley Periodicals, Inc. J Graph Theory 58:251‐260, 2008     

Connectivity properties of locally semicomplete digraphs

It is shown that every k-connected locally semicomplete digraph D with minimum outdegree at least 2k and minimum indegree at least 2k ? 2 has at least m = max{2, k} vertices x₁, x₂, ?, x_m such that D ? x_i is k-connected for i = 1, 2, ?, m.     

Efficient total domination in digraphs

We generalize the concept of efficient total domination from graphs to digraphs. An efficiently total dominating set X of a digraph D is a vertex subset such that every vertex of D has exactly one predecessor in X. We study graphs that permit an orientation having such a set and give complexity results and characterizations. Furthermore, we study the computational complexity of the (weighted) efficient total domination problem for several digraph classes. In particular we deal with most of the common generalizations of tournaments, like locally semicomplete and arc-locally semicomplete digraphs.     

On cycles in regular 3-partite tournaments

The vertex set of a digraph D is denoted by V(D). A c-partite tournament is an orientation of a complete c-partite graph. In 1991, Jian-zhong Wang conjectured that every arc of a regular 3-partite tournament D is contained in directed cycles of all lengths 3,6,9,…,|V(D)|. This conjecture is not valid, because for each integer t with 3?t?|V(D)|, there exists an infinite family of regular 3-partite tournaments D such that at least one arc of D is not contained in a directed cycle of length t.In this paper, we prove that every arc of a regular 3-partite tournament with at least nine vertices is contained in a directed cycle of length m, m+1, or m+2 for 3?m?5, and we conjecture that every arc of a regular 3-partite tournament is contained in a directed cycle of length m, (m+1), or (m+2) for each m∈{3,4,…,|V(D)|-2}.It is known that every regular 3-partite tournament D with at least six vertices contains directed cycles of lengths 3, |V(D)|-3, and |V(D)|. We show that every regular 3-partite tournament D with at least six vertices also has a directed cycle of length 6, and we conjecture that each such 3-partite tournament contains cycles of all lengths 3,6,9,…,|V(D)|.     

Cycles with a given number of vertices from each partite set in regular multipartite tournaments

If x is a vertex of a digraph D, then we denote by d ⁺(x) and d ⁻(x) the outdegree and the indegree of x, respectively. A digraph D is called regular, if there is a number p ∈ ℕ such that d ⁺(x) = d ⁻(x) = p for all vertices x of D. A c-partite tournament is an orientation of a complete c-partite graph. There are many results about directed cycles of a given length or of directed cycles with vertices from a given number of partite sets. The idea is now to combine the two properties. In this article, we examine in particular, whether c-partite tournaments with r vertices in each partite set contain a cycle with exactly r − 1 vertices of every partite set. In 1982, Beineke and Little [2] solved this problem for the regular case if c = 2. If c ⩾ 3, then we will show that a regular c-partite tournament with r ⩾ 2 vertices in each partite set contains a cycle with exactly r − 1 vertices from each partite set, with the exception of the case that c = 4 and r = 2.     

The structure of strong arc-locally in-semicomplete digraphs

A digraph is arc-locally in-semicomplete if for any pair of adjacent vertices $x,y$, every in-neighbor of $x$ and every in-neighbor of $y$ either are adjacent or are the same vertex. In this paper, we study the structure of strong arc-locally in-semicomplete digraphs and prove that a strong arc-locally in-semicomplete digraph is either arc-locally semicomplete or in a special class of digraphs. Using this structural characterization, we show that a 2-strong arc-locally in-semicomplete digraph is arc-locally semicomplete and a conjecture of Bang-Jensen is true.     

Finding cheapest cycles in vertex-weighted quasi-transitive and extended semicomplete digraphs

We consider the problem of finding a minimum cost cycle in a digraph with real-valued costs on the vertices. This problem generalizes the problem of finding a longest cycle and hence is NP-hard for general digraphs. We prove that the problem is solvable in polynomial time for extended semicomplete digraphs and for quasi-transitive digraphs, thereby generalizing a number of previous results on these classes. As a byproduct of our method we develop polynomial algorithms for the following problem: Given a quasi-transitive digraph D with real-valued vertex costs, find, for each j=1,2,…,|V(D)|, j disjoint paths P₁,P₂,…,P_j such that the total cost of these paths is minimum among all collections of j disjoint paths in D.     

Locally semicomplete digraphs that are complementary m-pancyclic

If A and B are two subdigraphs of D, then we denote by d_D (A, B) the distance between A and B. Let D be a 2-connected locally semicomplete digraph on n ≥ 6 vertices. If S is a minimum separating set of D and then m = max{3, d + 2} ≤ n/2 and D contains two vertex-disjoint dicycles of lengths t and n − t for every integer t satisfying m ≤ t ≤ n/2, unless D is a member of a family of locally semicomplete digraphs. This result extends those of Reid (Ann. Discrete Math. 27 (1985), 321–334) and Song (J. Combin. Theory B 57 (1993), 18–25) for tournaments, and it confirms two conjectures of Bang-Jensen (Discrete Math. 100 (1992), 243–265. © 1996 John Wiley & Sons, Inc.     

Complementary cycles in regular multipartite tournaments, where one cycle has length five

The vertex set of a digraph D is denoted by V(D). A c-partite tournament is an orientation of a complete c-partite graph.In 1999, Yeo conjectured that each regular c-partite tournament D with c≥4 and |V(D)|≥10 contains a pair of vertex disjoint directed cycles of lengths 5 and |V(D)|−5. In this paper we shall confirm this conjecture using a computer program for some cases.     

超欧拉扩展有向图

A digraph D is supereulerian if D has a spanning eulerian subdigraph. BangJensen and Thomass′e conjectured that if the arc-strong connectivity λ(D) of a digraph D is not less than the independence number α(D), then D is supereulerian. In this paper, we prove that if D is an extended cycle, an extended hamiltonian digraph, an arc-locally semicomplete digraph, an extended arc-locally semicomplete digraph, an extension of two kinds of eulerian digraph, a hypo-semicomplete digraph or an extended hypo-semicomplete digraph satisfyingλ(D) ≥α(D), then D is supereulerian.     

On the structure of strong 3-quasi-transitive digraphs

In this paper, D=(V(D),A(D)) denotes a loopless directed graph (digraph) with at most one arc from u to v for every pair of vertices u and v of V(D). Given a digraph D, we say that D is 3-quasi-transitive if, whenever u→v→w→z in D, then u and z are adjacent or u=z. In Bang-Jensen (2004) [3], Bang-Jensen introduced 3-quasi-transitive digraphs and claimed that the only strong 3-quasi-transitive digraphs are the strong semicomplete digraphs and strong semicomplete bipartite digraphs. In this paper, we exhibit a family of strong 3-quasi-transitive digraphs distinct from strong semicomplete digraphs and strong semicomplete bipartite digraphs and provide a complete characterization of strong 3-quasi-transitive digraphs.