@article{10_1017_S0017089500006121,
     author = {Lehner, J.},
     title = {Diophantine approximation on {Hecke} groups},
     journal = {Glasgow mathematical journal},
     pages = {117--127},
     year = {1985},
     volume = {26},
     doi = {10.1017/S0017089500006121},
     url = {http://geodesic.mathdoc.fr/articles/10.1017/S0017089500006121/}
}