@article{JEMS_2018_20_5_a1,
     author = {Tobias Finis and Erez Lapid},
     title = {An approximation principle for congruence subgroups},
     journal = {Journal of the European Mathematical Society},
     pages = {1075--1138},
     publisher = {mathdoc},
     volume = {20},
     number = {5},
     year = {2018},
     doi = {10.4171/jems/783},
     url = {http://geodesic.mathdoc.fr/articles/10.4171/jems/783/}
}