Congruence subgroup conjecture for groups generated by root unipotents

Fix a nonzero rational number q=stQ(4,4)q=\tfrac{s}{t}\in\mathbb{Q}\cap(-4,4) in lowest terms, with gcd(s,t)=1\gcd(s,t)=1. Let Γq\Gamma_q be the group generated by the root unipotents associated to qq, and let

Γ1(t)(s)={(abcd)SL(2,Z[1t]):atdt1(mods),ct0(mods)},\Gamma_1^{(t)}(s)=\left\{\begin{pmatrix} a&b\\ c&d\end{pmatrix}\in\operatorname{SL}\left(2,\mathbb{Z}\left[\tfrac{1}{t}\right]\right): a\equiv_t d\equiv_t1\pmod s,\quad c\equiv_t0\pmod s\right\},

where atb(mods)a\equiv_t b\pmod s means absZ[1t]a-b\in s\mathbb{Z}[\tfrac{1}{t}]. Congruence subgroup conjecture. For every nonzero q=stQ(4,4)q=\tfrac{s}{t}\in\mathbb{Q}\cap(-4,4), the group Γq\Gamma_q is equal to the congruence subgroup Γ1(t)(s)\Gamma_1^{(t)}(s). This conjecture proposes an exact arithmetic description of the groups generated by root unipotents, extending the known arithmeticity results and giving a precise form for the congruence subgroups arising in this setting.

Sources & referencesView supporting material

Primary source

Yanlong Hao, “Groups Generated by Root Unipotents: Higher-rank and rank-one”, arXiv:2607.04580 (2026).

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