[1] J. Aczél: 
Lectures on Functional Equations and Their Applications. New York and London: Academic Press, 1966. 
MR 0208210[3] M. Cyparissos Stephanos: 
Sur une categorie d'èquations fonctionnelles. Rend. Circ. Mat. Palermo 18 (1904), 360-362. 
DOI 10.1007/BF03014110[4] H. Gauchman, L. A. Rubel: 
Sums of products of functions of $x$ times functions of $y$. Linear Algebra Appl. 125 (1989), 19-63. 
MR 1024482 | 
Zbl 0695.26007[5] F. Neuman: 
Functions of two variables and matrices involving factorization. C. R. Math. Rep. Acad. Sci. Canada. 3 (1981), 7-11. 
MR 0608675[6] F. Neuman: 
Factorizations of matrices and functions of two variables. Czechoslovak Math. J. 32 (107) (1982), 582-588. 
MR 0682133 | 
Zbl 0517.15012[8] T. M. Rassias: 
A criterion for a function to be represented as a sum of products of factors. Bull. Inst. Math. Acad. Sinica 14 (1986), 377-382. 
MR 0885367 | 
Zbl 0643.26009