@article{SM_2008_199_7_a3,
     author = {V. V. Golovchanskii and M. N. Smotrov},
     title = {An explicit formula for the number of classes of primitive hyperbolic elements in the group $\Gamma_0(N)$},
     journal = {Sbornik. Mathematics},
     pages = {1009--1031},
     year = {2008},
     volume = {199},
     number = {7},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_2008_199_7_a3/}
}