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@article{MV_1969_21_3_a12, author = {R. Daci\'c}, title = {The cosine functional equation for groups}, journal = {Matemati\v{c}ki vesnik}, pages = {339 }, publisher = {mathdoc}, volume = {21}, number = {3}, year = {1969}, url = {http://geodesic.mathdoc.fr/item/MV_1969_21_3_a12/} }
R. Dacić. The cosine functional equation for groups. Matematički vesnik, Tome 21 (1969) no. 3, p. 339 . http://geodesic.mathdoc.fr/item/MV_1969_21_3_a12/