1. Baker, J.,On Quadratic Functionals Continuous Along Rays, Glasnik Mat.3 (23), 215–229 (1968).
2. Dacić, R.,The Cosine Functional Equation for Groups, Mat. Vesnik6 (21), 339–342 (1969).
3. Fréchet, M.,Sur la définition axiomatique d'une classe d'espaces vectoriels distanciés applicables vectoriellement sur l'espace de Hilbert, Ann. Math. (IIS)36, 705–718 (1935).
4. Hosszú, M.,Some Remarks on the Cosine Functional Equation, Publ. Math. Debrecen16, 93–98 (1969).
5. Hosszú, M.,A Remark on the Square Norm, Aequationes Math.2, 190–193 (1969).