Halin's end degree conjecture

Let GG be a graph and let an end of GG be an equivalence class of rays, where two rays are equivalent if there are infinitely many vertex-disjoint paths between them. The degree of an end is the maximum cardinality of a collection of pairwise disjoint rays in that equivalence class. Given a set of disjoint equivalent rays in GG, a ray graph is a graph whose vertex set is that set of rays, with an edge RSRS whenever there are infinitely many disjoint independent RR--SS paths in GG. A ray graph for an end is a connected ray graph on a degree-witnessing subset of that end.

Halin's conjecture. Every graph contains ray graphs for all its ends.

For ends of finite degree, the assertion is straightforward, and it is also known for ends of countably infinite degree, where the ray graph can be chosen as a ray using Halin's grid theorem. The conjecture remains open for ends of uncountable degree.

Sources & referencesView supporting material

Primary source

Stefan Geschke, Jan Kurkofka, Ruben Melcher and Max Pitz, “Halin's end degree conjecture”, arXiv:2010.10394 (2020).

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