Discreteness conjecture for commensurators of subgroups of lattices in PSL2(R)\mathrm{PSL}_2(\mathbb{R})

From papers

Let H<PSL2(Z)H<\mathrm{PSL}_2(\mathbb{Z}) be a finite index subgroup with b1(H)1b_1(H)\geq 1. Let Φ(H)\Phi(H) denote the subgroup introduced in the paper, and let KK be an infinite index normal subgroup of a lattice Γ<PSL2(R)\Gamma<\mathrm{PSL}_2(\mathbb{R}) such that K=|K|=\infty. For a subgroup L<PSL2(R)L<\mathrm{PSL}_2(\mathbb{R}), write CommPSL2(R)(L)\operatorname{Comm}_{\mathrm{PSL}_2(\mathbb{R})}(L) for its commensurator.

Commensurator discreteness conjecture. The group CommPSL2(R)(Φ(H))\operatorname{Comm}_{\mathrm{PSL}_2(\mathbb{R})}(\Phi(H)) is discrete provided that HΦ(H)H\neq\Phi(H). More generally,

CommPSL2(R)(K) is discrete.\operatorname{Comm}_{\mathrm{PSL}_2(\mathbb{R})}(K)\text{ is discrete}.

These statements would extend the paper's discreteness theorem beyond the hypothesis that HH is contained as a normal subgroup of a principal congruence subgroup. The source presents them as expected generalizations, and does not give evidence that they have been resolved.

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Sources & referencesView supporting material

Primary source

Thomas Koberda and Mahan Mj, “Commutators, commensurators, and PSL_2(Z)”, arXiv:1810.11429 (2021).

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