1. The cosine functional equation for groups;Dacić R.;Math. Vesnik,1969
2. Wilson’s functional equation in Banach algebras;Fechner;Acta Sci. Math. (Szeged),2009
3. The functional equation f(xy)+f(xy−1) = 2f(x)f(y) for groups;Kannappan Pl.;Proc. Amer. Math. Soc.,1968
4. On some functional equations in Banach spaces;Kurepa S.;Studia Math.,1960
5. A cosine functional equation in Banach algebras;Kurepa S.;Acta Sci. Math. (Szeged),1962