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Keywords:
oriented graph; cycle; minimum semidegree
oriented graph; cycle; minimum semidegree
Summary:
An orientation of a simple graph is referred to as an oriented graph. Caccetta and Häggkvist conjectured that any digraph on $n$ vertices with minimum outdegree $d$ contains a directed cycle of length at most $\lceil n / d\rceil $. In this paper, we consider short cycles in oriented graphs without directed triangles. Suppose that $\alpha _0$ is the smallest real such that every $n$-vertex digraph with minimum outdegree at least $\alpha _0n$ contains a directed triangle. Let $\epsilon < {(3-2\alpha _0)}/{(4-2\alpha _0)}$ be a positive real. We show that if $D$ is an oriented graph without directed triangles and has minimum outdegree and minimum indegree at least $(1/{(4-2\alpha _0)}+\epsilon )|D|$, then each vertex of $D$ is contained in a directed cycle of length $l$ for each $4\le l< {(4-2\alpha _0)\epsilon |D|}/{(3-2\alpha _0)}+2$.
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