@article{AM2_1978__18_136785,
     author = {Stefan Czerwik},
     title = {Solutions of a system of functional equations in some class functions.},
     journal = {Aequationes mathematicae},
     pages = {289--295},
     publisher = {mathdoc},
     volume = {18},
     year = {1978},
     zbl = {0406.39009},
     url = {http://geodesic.mathdoc.fr/item/AM2_1978__18_136785/}
}