Characterisation of wild auxiliary arcs between vertices

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Let GG be a graph and let xx and yy be distinct vertices of GG. Write S  →(G−xy)\overset{{}_{\;\rightarrow}}{S}(G-xy) for the partially ordered set of oriented vertex separators of G−xyG-xy, let T2T_2 be the infinite binary tree, and let ⟨ ⋅ ⟩ ⁣:⋃n<ω2n→I\langle\,\cdot\,\rangle\colon\bigcup_{n<\omega}2^n\to\mathbb{I} map each binary sequence to its naturally corresponding value in the unit interval. An auxiliary arc from xx to yy is called wild when it has the wildness property defined in the paper, and % \mathrel{\vcenter{\offinterlineskip \text{\sim}\vskip-.35ex\text{\sim}\vskip-.35ex\text{\sim}}} and ∼\sim denote the paper's auxiliary equivalence relations. Wild-arc characterisation conjecture. For every graph GG and every two distinct vertices xx and yy of GG, the following are equivalent: (1) x∼yx\sim y and every auxiliary arc from xx to yy is wild; (2) there \exists a countable \subset WW of S  →(G−xy)\overset{{}_{\;\rightarrow}}{S}(G-xy) such that: (a) every (A,B)∈W(A,B)\in W satisfies x∈A∖Bx\in A\setminus B and y∈B∖Ay\in B\setminus A; (b) the \partial ordering ≤\le of S  →(G−xy)\overset{{}_{\;\rightarrow}}{S}(G-xy) induces the ordering of the rationals on WW; and (c) there \exists a bijection φ ⁣:T2→W\varphi\colon T_2\to W satisfying φ(a)≤φ(b)\varphi(a)\le\varphi(b) if and only if ⟨a⟩≤⟨b⟩\langle a\rangle\le\langle b\rangle. If φ^ ⁣:T2→X\hat{\varphi}\colon T_2\to\mathcal{X} maps each vertex of T2T_2 to the vertex separator of φ(a)\varphi(a), n(a)∈Nn(a)\in\mathbb{N} is the level containing aa, and XaX_a is the set of all vertices in the first n(a)n(a) levels of T2T_2 other than aa, then for every a∈V(T2)a\in V(T_2) and every auxiliary arc AA from xx to yy, the set φ^(a)∖⋃b∈Xaφ^(b)\hat{\varphi}(a)\setminus\bigcup_{b\in X_a}\hat{\varphi}(b) meets AA; or (3) x\not% \mathrel{\vcenter{\offinterlineskip \text{\sim}\vskip-.35ex\text{\sim}\vskip-.35ex\text{\sim}}} y.

The conjecture seeks a structural characterisation of when auxiliary arcs are wild, using a countable rationally ordered family of separators arranged like the infinite binary tree. The supplied passage raises this as an outlook question and gives no evidence of resolution, so its status is open.

References

Primary source

Jan Kurkofka, “On the tangle compactification of infinite graphs”, arXiv:1908.10212 (2019).

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