Cauchy subsequences and hulls of sequences in labeled trees

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Let TT be a tree, and let (vn)n∈N(v_n)_{n\in\mathbb{N}} be a sequence of distinct vertices of TT. For a set of vertices, its hull is the corresponding minimal connected subgraph. A labeling l:V(T)→Rl:V(T)\to\mathbb{R} is non-degenerate in the sense used for the ultrametric induced by ll.

Cauchy-hull conjecture. The following conditions are equivalent:

  1. The hull of the range set of (vn)n∈N(v_n)_{n\in\mathbb{N}} is a union of a ray with some finite tree.
  2. For every non-degenerate l:V(T)→Rl:V(T)\to\mathbb{R}, the existence of a Cauchy subsequence of (vn)n∈N(v_n)_{n\in\mathbb{N}} implies that (vn)n∈N(v_n)_{n\in\mathbb{N}} is itself a Cauchy sequence.

This conjecture links the geometry of the hull of a sequence's range to the behavior of Cauchy subsequences in the ultrametrics induced by labelings. Its resolution status is not specified in the source.

References

Primary source

Oleksiy Dovgoshey and Mehmet Küçükaslan, “Labeled trees generating complete, compact, and discrete ultrametric spaces”, arXiv:2101.00626 (2022).

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