@article{10_4064_ap_10_1_1_5,
     author = {S. Kurepa},
     title = {On the functional equation $f(x+y)f(x-y)=f^2(x)-f^2 (y)$},
     journal = {Annales Polonici Mathematici},
     pages = {1--5},
     publisher = {mathdoc},
     volume = {10},
     number = {1},
     year = {1961},
     doi = {10.4064/ap-10-1-1-5},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/ap-10-1-1-5/}
}