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

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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 Comm⁡PSL2(R)(L)\operatorname{Comm}_{\mathrm{PSL}_2(\mathbb{R})}(L) for its commensurator.

Commensurator discreteness conjecture. The group Comm⁡PSL2(R)(Φ(H))\operatorname{Comm}_{\mathrm{PSL}_2(\mathbb{R})}(\Phi(H)) is discrete provided that H≠Φ(H)H\neq\Phi(H). More generally,

Comm⁡PSL2(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.

References

Primary source

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

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