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

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Let CkC_k be a directed cycle on kk vertices, with k,n∈Nk,n\in\mathbb{N} and k≥3k\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/log⁡nc 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.

References

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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