@article{10_2307_2661374,
     author = {J. Lewis and D. Zagier},
     title = {Period functions for {Maass} wave forms. {I.}},
     journal = {Annals of mathematics},
     year = {2001},
     volume = {153},
     number = {1},
     doi = {10.2307/2661374},
     zbl = {1061.11021},
     url = {http://geodesic.mathdoc.fr/articles/10.2307/2661374/}
}