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@article{MV_1971_23_1_a38, author = {PL. Kannappan}, title = {A note on cosine functional equation for groups}, journal = {Matemati\v{c}ki vesnik}, pages = {317 }, publisher = {mathdoc}, volume = {23}, number = {1}, year = {1971}, url = {http://geodesic.mathdoc.fr/item/MV_1971_23_1_a38/} }
PL. Kannappan. A note on cosine functional equation for groups. Matematički vesnik, Tome 23 (1971) no. 1, p. 317 . http://geodesic.mathdoc.fr/item/MV_1971_23_1_a38/