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