Congruence Subgroup Property for nilpotent groups and subsurface subgroups of Mapping Class Groups

Abstract

We prove the Congruence Subgroup Property for two families of subgroups of Mapping Class Groups of finite-type surfaces. The first one is related to nilpotent quotients of the fundamental group and Johnson filtration, and along the way we give an elementary proof of a theorem of Ben-Ezra-Lubotzky. The second family consists of certain geometric subgroups obtained via subsurface inclusions.

20 pages, 4 figures; updated with some important references