Geodesic
A Functional Equation with Differences
Zbornik radova, Tome 1 (1976) no. 9, p. 49 
Gyula Maksa. A Functional Equation with Differences. Zbornik radova, Tome 1 (1976) no. 9, p. 49 . http://geodesic.mathdoc.fr/item/ZR_1976_1_9_a7/
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     author = {Gyula Maksa},
     title = {A {Functional} {Equation} with {Differences}},
     journal = {Zbornik radova},
     pages = {49 },
     year = {1976},
     volume = {1},
     number = {9},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/ZR_1976_1_9_a7/}
}
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Voir la notice de l'article provenant de la source eLibrary of Mathematical Institute of the Serbian Academy of Sciences and Arts

Let $R$ denote the set of real number. Let us determine all functions $f:R\rightarrow R$ such that $\frac{f(tx+ty(-f(tx)}{f(tx)-f(tx-ty)}=\frac{f(x+y)-f(x)}{f(x)-f(x-y)}$ for all $x,y,t\in R,\;\;yt\neq 0$. This problem is due to P. Drṟeve{a}gilṟeve{a} [1].