The dichotomy conjecture for conjugacy of discrete subgroups of connected Lie groups

From papers

Let GG be a connected Lie group, let ath.E(G,D(G))ath.E(G,\mathcal{D}(G)) denote the conjugacy relation on the space D(G)\mathcal{D}(G) of discrete subgroups of GG, and call an equivalence relation essentially hyperfinite or essentially countable universal in the usual Borel-reducibility sense. Connected-Lie-group conjugacy dichotomy. Exactly one of the following holds:

  1. E(G,D(G))E(G,\mathcal{D}(G)) is essentially hyperfinite; or
  2. E(G,D(G))E(G,\mathcal{D}(G)) is essentially countable universal.

The proposed dichotomy is motivated by the result that a connected Lie group fails to contain a discrete nonabelian free subgroup exactly when it is a compact extension of a solvable subgroup. In particular, the conjecture predicts essential hyperfiniteness for conjugacy on discrete subgroups of compact-by-solvable Lie groups; the general dichotomy remains open.

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Primary source

Jeffrey Bergfalk and Iian B. Smythe, “Manifold classification from the descriptive viewpoint”, arXiv:2512.24996 (2025).

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