Cauchy subsequences and hulls of sequences in labeled trees

Let TT be a tree, and let (vn)nN(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)nN(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)nN(v_n)_{n\in\mathbb{N}} implies that (vn)nN(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.

Sources & referencesView supporting material

Primary source

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

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