Out-arc pancyclicity of vertices in tournaments

Yao, Guo and Zhang [T. Yao, Y. Guo, K. Zhang, Pancyclic out-arcs of a vertex in a tournament, Discrete Appl. Math. 99 (2000) 245-249.] proved that every strong tournament contains a vertex u such that every out-arc of u is pancyclic. In this paper, we prove that every strong tournament with minimum out-degree at least two contains two such vertices. Yeo [A. Yeo, The number of pancyclic arcs in a k-strong tournament, J. Graph Theory 50 (2005) 212-219.] conjectured that every 2-strong tournament has three distinct vertices {x,y,z}, such that every arc out of x,y and z is pancyclic. In this paper, we also prove that Yeo’s conjecture is true.     

Edge‐disjoint Hamiltonian cycles in hypertournaments

We introduce a method for reducing k‐tournament problems, for k ≥ 3, to ordinary tournaments, that is, 2‐tournaments. It is applied to show that a k‐tournament on n ≥ k + 1 + 24d vertices (when k ≥ 4) or on n ≥ 30d + 2 vertices (when k = 3) has d edge‐disjoint Hamiltonian cycles if and only if it is d‐edge‐connected. Ironically, this is proved by ordinary tournament arguments although it only holds for k ≥ 3. We also characterizatize the pancyclic k‐tournaments, a problem posed by Gutin and Yeo.(Our characterization is slightly incomplete in that we prove it only for n large compared to k.). © 2005 Wiley Periodicals, Inc. J Graph Theory     

Each 3-strong Tournament Contains 3 Vertices Whose Out-arcs Are Pancyclic

An arc in a tournament T with n ≥ 3 vertices is called pancyclic, if it is in a cycle of length k for all 3 ≤ k ≤ n. Yeo (Journal of Graph Theory, 50 (2005), 212–219) proved that every 3-strong tournament contains two distinct vertices whose all out-arcs are pancyclic, and conjectured that each 2-strong tournament contains 3 such vertices. In this paper, we confirm Yeo’s conjecture for 3-strong tournaments. The author is an associate member of “Graduiertenkolleg: Hierarchie und Symmetrie in mathematischen Modellen (DFG)” at RWTH Aachen University, Germany.     

The number of vertices whose out-arcs are pancyclic in a 2-strong tournament

An arc going out from a vertex x in a digraph is called an out-arc of x. Yao et al. [Discrete Appl. Math. 99 (2000) 245-249] proved that every strong tournament contains a vertex x such that all out-arcs of x are pancyclic. Recently, Yeo [J. Graph Theory 50 (2005) 212-219] proved that each 3-strong tournament contains two such vertices. In this paper, we confirm that Yeo's result is also true for 2-strong tournaments. Our proof implies a polynomial algorithm to find two such vertices.     

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.     

The structure of 4‐strong tournaments containing exactly three out‐arc pancyclic vertices

Yao et al. (Discrete Appl Math 99 (2000), 245–249) proved that every strong tournament contains a vertex u such that every out‐arc of u is pancyclic and conjectured that every k‐strong tournament contains k such vertices. At present, it is known that this conjecture is true for k = 1, 2, 3 and not true for k?4. In this article, we obtain a sufficient and necessary condition for a 4‐strong tournament to contain exactly three out‐arc pancyclic vertices, which shows that a 4‐strong tournament contains at least four out‐arc pancyclic vertices except for a given class of tournaments. Furthermore, our proof yields a polynomial algorithm to decide if a 4‐strong tournament has exactly three out‐arc pancyclic vertices.     

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.     

Arc-disjoint hamiltonian paths in non-round decomposable local tournaments

Thomassen proved that a strong tournament $T$ has a pair of arc-disjoint Hamiltonian paths with distinct initial vertices and distinct terminal vertices if and only if $T$ is not an almost transitive tournament of odd order, where an almost transitive tournament is obtained from a transitive tournament with acyclic ordering $u_1,u_2,\dots ,u_n$ (i.e., $u_i\to u_j$ for all $1\le i<j\le n$) by reversing the arc $u_1{u}_n$. A digraph $D$ is a local tournament if for every vertex $x$ of $D$, both the out-neighbors and the in-neighbors of $x$ induce tournaments. Bang-Jensen, Guo, Gutin and Volkmann split local tournaments into three subclasses: the round decomposable; the non-round decomposable which are not tournaments; the non-round decomposable which are tournaments. In 2015, we proved that every 2-strong round decomposable local tournament has a Hamiltonian path and a Hamiltonian cycle which are arc-disjoint if and only if it is not the second power of an even cycle. In this paper, we discuss the arc-disjoint Hamiltonian paths in non-round decomposable local tournaments, and prove that every 2-strong non-round decomposable local tournament contains a pair of arc-disjoint Hamiltonian paths with distinct initial vertices and distinct terminal vertices. This result combining with the one on round decomposable local tournaments extends the above-mentioned result of Thomassen to 2-strong local tournaments.     

A polynomial algorithm for the Hamiltonian cycle problem in semicomplete multipartite digraphs

We describe a polynomial algorithm for the Hamiltonian cycle problem for semicomplete multipartite digraphs. The existence of such an algorithm was conjectured in G. Gutin, Paths and cycles in digraphs. Ph. D. thesis, Tel Aviv Univ., 1993. (see also G. Gutin, J Graph Theory 19 (1995) 481–505). © 1998 John Wiley & Sons, Inc. J. Graph Theory 29: 111–132, 1998     

多部竞赛图中包含某条弧的圈

多部竞赛图或n部竞赛图是指一个完全n部无向图的定向图.2007年Volkmann证明了每个强连通的n部竞赛图(n≥3)至少存在一条弧它包含在从3到n的每个长度的圈中.在此基础上给出了强连通n部竞赛图中存在一条弧它包含在从3到n+1的每个长度的圈中的一个充分条件,并举例说明该条件在某种意义上的最佳可能性.     

On k‐ary spanning trees of tournaments

It is well known that every tournament contains a Hamiltonian path, which can be restated as that every tournament contains a unary spanning tree. The purpose of this article is to study the general problem of whether a tournament contains a k‐ary spanning tree. In particular, we prove that, for any fixed positive integer k, there exists a minimum number h(k) such that every tournament of order at least h(k) contains a k‐ary spanning tree. The existence of a Hamiltonian path for any tournament is the same as h(1) = 1. We then show that h(2) = 4 and h(3) = 8. The values of h(k) remain unknown for k ≥ 4. © 1999 John & Sons, Inc. J Graph Theory 30: 167–176, 1999     

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     

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.     

Characterization and linear-time detection of minimal obstructions to concave-round graphs and the circular-ones property

A graph is concave-round if its vertices can be circularly enumerated so that the closed neighborhood of each vertex is an interval in the enumeration. In this study, we give a minimal forbidden induced subgraph characterization for the class of concave-round graphs, solving a problem posed by Bang-Jensen, Huang, and Yeo [SIAM J. Discrete Math., 13 (2000), pp. 179–193]. In addition, we show that it is possible to find one such forbidden induced subgraph in linear time in any given graph that is not concave-round. As part of the analysis, we obtain characterizations by minimal forbidden submatrices for the circular-ones property for rows and for the circular-ones property for rows and columns and show that, also for both variants of the property, one of the corresponding forbidden submatrices can be found (if present) in any given matrix in linear time. We make some final remarks regarding connections to some classes of circular-arc graphs.     

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     

On arc-traceable local tournaments

A digraph D is arc-traceable if for every arc xy of D, the arc xy belongs to a directed Hamiltonian path of D. A local tournament is an oriented graph such that the negative neighborhood as well as the positive neighborhood of every vertex induces a tournament. It is well known that every tournament contains a directed Hamiltonian path and, in 1990, Bang-Jensen showed the same for connected local tournaments. In 2006, Busch, Jacobson and Reid studied the structure of tournaments that are not arc-traceable and consequently gave various sufficient conditions for tournaments to be arc-traceable. Inspired by the article of Busch, Jacobson and Reid, we develop in this paper the structure necessary for a local tournament to be not arc-traceable. Using this structure, we give sufficient conditions for a local tournament to be arc-traceable and we present examples showing that these conditions are best possible.     

Some 3‐connected 4‐edge‐critical non‐Hamiltonian graphs

Let γ(G) be the domination number of graph G, thus a graph G is k‐edge‐critical if γ (G) = k, and for every nonadjacent pair of vertices u and υ, γ(G + uυ) = k?1. In Chapter 16 of the book “Domination in Graphs—Advanced Topics,” D. Sumner cites a conjecture of E. Wojcicka under the form “3‐connected 4‐critical graphs are Hamiltonian and perhaps, in general (i.e., for any k ≥ 4), (k?1)‐connected, k‐edge‐critical graphs are Hamiltonian.” In this paper, we prove that the conjecture is not true for k = 4 by constructing a class of 3‐connected 4‐edge‐critical non‐Hamiltonian graphs. © 2005 Wiley Periodicals, Inc.     

On 2-factors with k components

In this paper we study the minimum degree condition for a Hamiltonian graph to have a 2-factor with k components. By proving a conjecture of Faudree et al. [A note on 2-factors with two components, Discrete Math. 300 (2005) 218-224] we show the following. There exists a real number ε>0 such that for every integer k?2 there exists an integer n₀=n₀(k) such that every Hamiltonian graph G of order n?n₀ with has a 2-factor with k components.     

Construction of hamiltonian cycles with a given spectrum of edge directions in an <Emphasis Type="Italic">n</Emphasis>-dimensional Boolean cube

The spectrum of a Hamiltonian cycle (of a Gray code) in an n-dimensional Boolean cube is the series a = (a ₁, ..., a _n ), where a _i is the number of edges of the ith direction in the cycle. The necessary conditions for the existence of a Gray code with the spectrum a are available: the numbers a _i are even and, for k = 1, ..., n, the sum of k arbitrary components of a is at least 2 ^k . We prove that there is some dimension N such that if the necessary condition on the spectrum is also sufficient for the existence of a Hamiltonian cycle with the spectrum in an N-dimensional Boolean cube then the conditions are sufficient for all dimensions n.     

Vertex‐pancyclicity of hypertournaments

A hypertournament or a k‐tournament, on n vertices, 2≤k≤n, is a pair T=(V, E), where the vertex set V is a set of size n and the edge set E is the collection of all possible subsets of size k of V, called the edges, each taken in one of its k! possible permutations. A k‐tournament is pancyclic if there exists (directed) cycles of all possible lengths; it is vertex‐pancyclic if moreover the cycles can be found through any vertex. A k‐tournament is strong if there is a path from u to v for each pair of distinct vertices u and v. A question posed by Gutin and Yeo about the characterization of pancyclic and vertex‐pancyclic hypertournaments is examined in this article. We extend Moon's Theorem for tournaments to hypertournaments. We prove that if k≥8 and n≥k + 3, then a k‐tournament on n vertices is vertex‐pancyclic if and only if it is strong. Similar results hold for other values of k. We also show that when n≥7, k≥4, and n≥k + 2, a strong k‐tournament on n vertices is pancyclic if and only if it is strong. The bound n≥k+ 2 is tight. We also find bounds for the generalized problem when we extend vertex‐pancyclicity to require d edge‐disjoint cycles of each possible length and extend strong connectivity to require d edge‐disjoint paths between each pair of vertices. Our results include and extend those of Petrovic and Thomassen. © 2009 Wiley Periodicals, Inc. J Graph Theory 63: 338–348, 2010