@article{IM2_2019_83_5_a6,
     author = {D. A. Popov},
     title = {The discrete spectrum of the {Laplace} operator on the fundamental domain of the modular group and the {Chebyshev} psi-function},
     journal = {Izvestiya. Mathematics },
     pages = {1066--1079},
     publisher = {mathdoc},
     volume = {83},
     number = {5},
     year = {2019},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/IM2_2019_83_5_a6/}
}