@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/}
}