Multi-separation star refinement conjecture

Let (U,,,,)(\vec{U},\leq,*,\vee,\wedge) be a distributive universe of separations with an order function, and let {xi:i[n]}\{\overleftarrow{x}_i:i\in[n]\} be a set of separations in U\vec{U}. A set of separations is a star if its oriented separations point pairwise towards one another in the standard sense, and ui\vec{u}_i is linked to xi\vec{x}_i when the two separations satisfy the linking relation used in the preceding lemma.

Multi-separation star refinement conjecture. There exists a set of separations {ui:i[n]}\{\overleftarrow{u}_i:i\in[n]\} such that:

  • {ui:i[n]}\{\overleftarrow{u}_i:i\in[n]\} is a star;
  • ui\vec{u}_i is linked to xi\vec{x}_i for all i[n]i\in[n];
  • uixi|\overleftarrow{u}_i|\leq|\overleftarrow{x}_i| for all i[n]i\in[n];
  • ui=xijiuj\overleftarrow{u}_i=\overleftarrow{x}_i\bigwedge_{j\neq i}\vec{u}_j for all i[n]i\in[n].

This would extend the preceding two-separation lemma to an arbitrary finite set of separations. The text calls it tempting rather than established, and no proof or resolution is supplied here.

Sources & referencesView supporting material

Primary source

Joshua Erde, “Refining a Tree-Decomposition which Distinguishes Tangles”, arXiv:1512.02499 (2017).

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