The dichotomy conjecture for conjugacy of discrete subgroups of connected Lie groups
The dichotomy conjecture for conjugacy of discrete subgroups of connected Lie groups
Let be a connected Lie group, let denote the conjugacy relation on the space of discrete subgroups of , 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:
- is essentially hyperfinite; or
- 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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Sources & referencesView supporting material
Primary source
Jeffrey Bergfalk and Iian B. Smythe, “Manifold classification from the descriptive viewpoint”, arXiv:2512.24996 (2025).
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