@article{JSFU_2018_11_1_a9,
     author = {Vladimir A. Koibaev},
     title = {On a question about generalized congruence subgroups},
     journal = {\v{Z}urnal Sibirskogo federalʹnogo universiteta. Matematika i fizika},
     pages = {66--69},
     publisher = {mathdoc},
     volume = {11},
     number = {1},
     year = {2018},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/JSFU_2018_11_1_a9/}
}