@article{AM2_1977__15_136633,
     author = {Ilie Corvei},
     title = {The cosine functional equation for nilpotent groups.},
     journal = {Aequationes mathematicae},
     pages = {99--106},
     publisher = {mathdoc},
     volume = {15},
     year = {1977},
     zbl = {0348.39004},
     url = {http://geodesic.mathdoc.fr/item/AM2_1977__15_136633/}
}