1. Shulman, E.: Functional equations of homological type, PhD Thesis. Moscow Institute of Electronics and Mathematics, Moscow (1994)
2. Shulman, E.: Group representations and stability of functional equations. J. London Math. Soc. 54(2), 111–120 (1996)
3. Stéphanos, C.: Sur une catégorie d’équations fonctionnelles. Rend. Circ. Mat. Palermo 18, 360–362 (1904)
4. Levi-Cività, T.: Sulle funzioni che ammettono una formula d’addizione del tipo
$$f(x+y)=\sum _{i=1}^n X_i(x)Y_i(y)$$
. Atti Accad. Naz. Lincei Rend. 22(5), 181–183 (1913)
5. Stäkel, P.: Sulla equazione funzionale
$$f(x+y)=\sum _{i=1}^n X_i(x)Y_i(y)$$
. Atti. Accad. Naz. Lincei Rend. 22(5), 392–393 (1913)