@article{ZNSL_1976_64_a7,
     author = {E. V. Dybkova},
     title = {Congruence subgroups of the symplectic group},
     journal = {Zapiski Nauchnykh Seminarov POMI},
     pages = {80--91},
     year = {1976},
     volume = {64},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/ZNSL_1976_64_a7/}
}