@article{AA_2011_23_4_a1,
     author = {V. V. Golovchanskii and M. N. Smotrov},
     title = {On the spectrum of the {Laplace--Beltrami} operator for congruence subgroups of the modular group},
     journal = {Algebra i analiz},
     pages = {31--39},
     publisher = {mathdoc},
     volume = {23},
     number = {4},
     year = {2011},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/AA_2011_23_4_a1/}
}