@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 },
     year = {1969},
     volume = {21},
     number = {3},
     url = {http://geodesic.mathdoc.fr/item/MV_1969_21_3_a12/}
}