Commit 2025-09-13 14:08 a840318b

feat(NumberTheory/ModularForms): Cusps of subgroups acting on upper half-plane (#29327)

• Define "arithmetic subgroups" of GL(2, R), as subgroups commensurable with the image of SL(2, Z)
• Add some notations and API to make it easier to work with images in GL(2, R) of subgroups of SL(2, Z)
• Define "cusps" of a subgroup of GL(2, R), as fixed points of parabolic elements on P^1(R).
• Prove that the cusps of an arithmetic subgroup are precisely P^1(Q).