The linear-backwards-edge conjecture for forbidden-cycle oriented graphs and digraphs
Let be a directed cycle on vertices, with and . Given a labelled digraph or oriented graph on vertices , 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:
- Almost all -free oriented graphs on vertices have backwards edges in a transitive-optimal ordering.
- If is even, then almost all -free digraphs on vertices have backwards edges in a transitive-optimal ordering.
The preceding theorem establishes only weaker bounds, namely between and in the oriented-graph case and, for even , 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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