The linear-backwards-edge conjecture for forbidden-cycle oriented graphs and digraphs

Let CkC_k be a directed cycle on kk vertices, with k,nNk,n\in\mathbb{N} and k3k\geq 3. Given a labelled digraph or oriented graph GG on vertices 1,,n1,\dots,n, a transitive-optimal ordering is an ordering of its vertices that minimises the number of backwards edges, where a backwards edge is directed from a vertex appearing later to one appearing earlier in the ordering.

Linear-backwards-edge conjecture. The following hold:

  1. Almost all CkC_k-free oriented graphs on nn vertices have Θ(n)\Theta(n) backwards edges in a transitive-optimal ordering.
  2. If kk is even, then almost all CkC_k-free digraphs on nn vertices have Θ(n)\Theta(n) backwards edges in a transitive-optimal ordering.

The preceding theorem establishes only weaker bounds, namely between cn/lognc n/\log n and αn2\alpha n^2 in the oriented-graph case and, for even kk, the same type of bounds in the digraph case. The conjecture strengthens these to linear order and remains open in the source.

Sources & referencesView supporting material

Primary source

Deryk Osthus, Daniela Kühn, Timothy Townsend and Yi Zhao, “On the structure of oriented graphs and digraphs with forbidden tournaments or cycles”, arXiv:1404.6178 (2015).

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