@article{10_1017_S0017089500004663,
     author = {Knopp, Marvin I.},
     title = {Rational period functions of the modular group {II}},
     journal = {Glasgow mathematical journal},
     pages = {185--197},
     year = {1981},
     volume = {22},
     number = {2},
     doi = {10.1017/S0017089500004663},
     url = {http://geodesic.mathdoc.fr/articles/10.1017/S0017089500004663/}
}