arXiv math/06056462006-05-24math.CO

A Summary of Problems and Results related to the Caccetta-Haggkvist Conjecture

Blair Dowling Sullivan

备注：: 14 pages, 0 figures

英文摘要：

This paper is an attempt to survey the current state of our knowledge on the Caccetta-Haggkvist conjecture and related questions. In January 2006 there was a workshop hosted by the American Institute of Mathematics in Palo Alto, on the Caccetta-Haggkvist conjecture, and this paper partly originated there, as a summary of the open problems and partial results presented at the workshop. This summary includes results and open problems related to Caccetta-Haggkvist, Seymour's Second Neighborhood Conjecture, the k/2 Conjecture (for nonedges), and connections with algebraic number theory through Cayley graphs, along with a number of other related topics.

机构、总结、中文摘要均由AI分析而来，可能存在错误，请以论文正文为准。

阅读原文

阅读 PDF ↗

arXiv 页面 ↗HTML 原文 ↗

本文内容

英文摘要相关论文

机构、总结、中文摘要均由AI分析而来，可能存在错误，请以论文正文为准。

继续探索

相关论文

Proof of the Caccetta-Haggkvist conjecture for digraphs with small independence number

For a digraph $G$ and $v \in V(G)$, let $δ^+(v)$ be the number of out-neighbors of $v$ in $G$. The Caccetta-Häggkvist conjecture states that for all $k \ge 1$, if $G$ is a digraph with $n = |V(G)|$ such that $δ^+(v) \ge n/k$ for all $v \in V(G)$, then G contains a directed cycle of length at most $k$. In [2], N. Lichiardopol proved that this conjecture is true for digraphs with independence number equal to two. In this paper, we generalize that result, proving that the conjecture is true for digraphs with independence number at most $(k+1)/2$.

2022-02-22math.CO

On the Caccetta-Haggkvist conjecture with forbidden subgraphs

The Caccetta-Haggkvist conjecture made in 1978 asserts that every orgraph on n vertices without oriented cycles of length <= l must contain a vertex of outdegree at most (n-1)/l. It has a rather elaborate set of (conjectured) extremal configurations. In this paper we consider the case l=3 that received quite a significant attention in the literature. We identify three orgraphs on four vertices each that are missing as an induced subgraph in all known extremal examples and prove the Caccetta-Haggkvist conjecture for orgraphs missing as induced subgraphs any of these orgraphs, along with cycles …

2012-08-17math.CO

Short cycles in digraphs and the Caccetta-Häggkvist conjecture

In the theory of digraphs, the study of cycles is a subject of great importance and has given birth to a number of deep questions such as the Behzad-Chartrand-Wall conjecture (1970) and its generalization, the Caccetta-Häggkvist conjecture (1978). Despite a lot of interest and efforts, the progress on these remains slow and mostly restricted to the solution of some special cases. In this note, we prove these conjectures for digraphs with girth is at least as large as their minimum out-degree and without short even cycles. More generally, we prove that if a digraph has sufficiently large girth …

2016-10-20math.CO

Rainbow triangles and the Caccetta-Häggkvist conjecture

A famous conjecture of Caccetta and Häggkvist is that in a digraph on $n$ vertices and minimum out-degree at least $\frac{n}{r}$ there is a directed cycle of length $r$ or less. We consider the following generalization: in an undirected graph on $n$ vertices, any collection of $n$ disjoint sets of edges, each of size at least $\frac{n}{r}$, has a rainbow cycle of length $r$ or less. We focus on the case $r=3$, and prove the existence of a rainbow triangle under somewhat stronger conditions than in the conjecture. For any fixed $k$ and large enough $n$, we determine the maximum number of edges …

2018-04-04math.CO

On Aharoni's rainbow generalization of the Caccetta-Häggkvist conjecture

For a digraph $G$ and $v \in V(G)$, let $δ^+(v)$ be the number of out-neighbors of $v$ in $G$. The Caccetta-Häggkvist conjecture states that for all $k \ge 1$, if $G$ is a digraph with $n = |V(G)|$ such that $δ^+(v) \ge k$ for all $v \in V(G)$, then G contains a directed cycle of length at most $\lceil n/k \rceil$. In [2], Aharoni proposes a generalization of this conjecture, that a simple edge-colored graph on $n$ vertices with $n$ color classes, each of size $k$, has a rainbow cycle of length at most $\lceil n/k \rceil$. In this paper, we prove this conjecture if each color class has size $Ω…

2021-10-01math.CO

The Caccetta-Haggkvist conjecture and additive number theory

The Caccetta-Haggkvist conjecture states that if G is a finite directed graph with at least n/k edges going out of each vertex, then G contains a directed cycle of length at most k. Hamidoune used methods and results from additive number theory to prove the conjecture for Cayley graphs and for vertex-transitive graphs. This expository paper contains a survey of results on the Caccetta-Haggkvist conjecture, and complete proofs of the conjecture in the case of Cayley and vertex-transitive graphs.

2006-03-20math.COmath.NT

更多相关论文收起更多论文

非均匀度与Caccetta-Häggkvist猜想的彩虹版本

Non-uniform degrees and rainbow versions of the Caccetta-Häggkvist conjecture

针对Caccetta-Häggkvist猜想，提出基于所有出度的有向围长上界，进而提出彩虹环版本的猜想，并在边集规模为1-2及边集为三角形的情形下取得验证结果。

2022-10-07math.CO

Aharoni 对 Caccetta-Häggkvist 猜想的彩虹推广的进一步逼近

Further approximations for Aharoni's rainbow generalization of the Caccetta-Häggkvist conjecture

本文研究 Aharoni 对 Caccetta-Häggkvist 猜想的彩虹推广，将每个颜色类大小的下界从 Ω(k log k) 改进为 Ω(k)，证明此时仍存在长度至多 ⌈n/k⌉ 的彩虹环，并进一步讨论了颜色数量条件放宽后的情形。

2023-08-10math.CO

Aharoni的Caccetta-Häggkvist猜想彩虹推广三角形式的改进界

Improved bounds for the triangle case of Aharoni's rainbow generalization of the Caccetta-Häggkvist conjecture

本文改进了Aharoni关于Caccetta-Häggkvist猜想彩虹推广的三角形式界，证明了(1.1077,1/3)、(1,0.3988)及(1.3481,1/4)等参数对是三角的。

2023-09-11math.CO

Strong connectivity and directed triangles in oriented graphs. Partial results on a particular case of the Caccetta-Häggkvist conjecture

A particular case of Caccetta-Häggkvist conjecture, says that a digraph of order $n$ with minimum out-degree at least $1/3n$ contains a directed cycle of length at most 3. Recently, Kral, Hladky and Norine proved that a digraph of order $n$ with minimum out-degree at least $0.3465n$ contains a directed cycle of length at most 3 (which currently is the best result). A weaker particular case says that a digraph of order $n$ with minimum semi-degree at least $1/3n$ contains a directed triangle. In a recent paper, by using the result of Kral et al, the author proved that for $β\geq 0.343545$, any …

2011-12-15math.CO

Short directed cycles in bipartite digraphs

The Caccetta-Häggkvist conjecture implies that for every integer $k\ge 1$, if $G$ is a bipartite digraph, with $n$ vertices in each part, and every vertex has out-degree more than $n/(k+1)$, then $G$ has a directed cycle of length at most $2k$. If true this is best possible, and we prove this for $k = 1,2,3,4,6$ and all $k\ge 224,539$. More generally, we conjecture that for every integer $k\ge 1$, and every pair of reals $α, β> 0$ with $kα+β>1$, if $G$ is a bipartite digraph with bipartition $(A,B)$, where every vertex in $A$ has out-degree at least $β|B|$, and every vertex in $B$ has out-degr…

2019-07-24math.CO

Counting flags in triangle-free digraphs

Motivated by the Caccetta-Haggkvist Conjecture, we prove that every digraph on n vertices with minimum outdegree 0.3465n contains an oriented triangle. This improves the bound of 0.3532n of Hamburger, Haxell and Kostochka. The main new tool we use in our proof is the theory of flag algebras developed recently by Razborov.

2016-02-22math.CO

匹配和三角形的族的短彩虹圈

Short rainbow cycles for families of matchings and triangles

本文研究了边集族中彩虹圈的长度问题，证明了在混合包含大小为2的匹配或三角形的情况下，彩虹圈长度渐近为O(log n)，并确定了引入单边时彩虹周长从线性变为对数的阈值比例。

2024-09-24cs.DMmath.CO

关于 $k$-反传递与 $m$-自由有向图的第二邻域问题的一个注记

A note on the second neighborhood problem for $k$-anti-transitive and $m$-free digraphs

本文证明：若有限有向图 $D$ 是 $k$-反传递且 $(k-4)$-自由的，则 $D$ 必存在 Seymour 顶点，从而在 7-反传递定向图上验证了 Caccetta-Haggkvist 猜想的一个特例，扩展了已知结果。

2024-05-28math.CO