Counting surface subgroups in cusped hyperbolic 3-manifolds

This paper establishes that the number of quasi-Fuchsian surface subgroups in finite-volume noncompact hyperbolic 3-manifolds grows asymptotically as (cg)2g(cg)^{2g}, a result that implies a similar lower bound for purely pseudo-Anosov surface subgroups in mapping class groups, while also demonstrating the existence of infinitely many conjugacy classes of surface subgroups with accidental parabolics.

Xiaolong Hans Han, Zhenghao Rao, Jia Wan2026-03-06🔢 math

Connected fundamental domains for congruence subgroups

This paper constructs canonical sets of right coset representatives for the congruence subgroups Γ0(N)\Gamma_0(N), Γ1(N)\Gamma_1(N), and Γ(N)\Gamma(N) to prove that their corresponding fundamental domains are connected, utilizing a study of the projective line P1(Z/NZ)\mathbb{P}^1(\mathbb{Z}/N\mathbb{Z}) and a multiplicity function MM that is shown to be one less than a more computable function WW.

Zhaohu Nie, C. Xavier Parent2026-03-05🔢 math