@article{10_1017_S001708950000940X,
     author = {Akbas, M. and Singerman, D.},
     title = {The normalizer of {\ensuremath{\Gamma}0(N)} in {PSL(2,} {R)}},
     journal = {Glasgow mathematical journal},
     pages = {317--327},
     year = {1990},
     volume = {32},
     number = {3},
     doi = {10.1017/S001708950000940X},
     url = {http://geodesic.mathdoc.fr/articles/10.1017/S001708950000940X/}
}