The conjectures on relation numbers and congruence structure of Δr/p\Delta_{r/p}

Let pp be prime and rZr\in\mathbb{Z} satisfy gcd(r,p)=1\gcd(r,p)=1, with r/pr/p admissible as defined in the source. Let Δr/p\Delta_{r/p} be generated by the two parabolic matrices AA and Qr/pQ_{r/p}, let UpU_p denote the diagonal matrix used in the source, let σp(r)\sigma_p(r) be the multiplicative order of pp modulo rr, and let J2(r)J_2(r) be the index quantity used there. The conjectures for admissible parameters. For every admissible r/pr/p, all seven assertions listed in the source hold: r/pr/p is a relation number; Δr/p\Delta_{r/p} has finite index and is a congruence subgroup; Δr/p=Γ1(p)(r)\Delta_{r/p}=\overline{\Gamma}_1^{(p)}(r) with index J2(r)J_2(r); Upσp(r)Δr/pU_p^{\sigma_p(r)}\in\Delta_{r/p}; r/pr/p is a strong relation number; Δr/p=Δr/pk\Delta_{r/p}=\Delta_{r/p^k} for every k1k\geq1; and SL2(Z[1p])=Δr/pSL2(Z)\operatorname{SL}_2\left(\mathbb{Z}[\frac1p]\right)=\Delta_{r/p}\cdot\operatorname{SL}_2(\mathbb{Z}). These are collected as conjectures at the end of the paper; their general status is unresolved.

Sources & referencesView supporting material

Primary source

Carl-Fredrik Nyberg-Brodda, “On congruence subgroups of SL_2(Z[1p]) generated by two parabolic elements”, arXiv:2312.11258 (2024).

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