Rassias, J.M.; Xu, T.Z.; Xu, W.X.
(Український математичний журнал, 2011)
We determine the general solution of the functional equation f(x+ky)+f(x−ky) = g(x+y)+g(x−y)+
+h(x)+h˜(y) for fixed integers k with k 6= 0, ±1 without assuming any regularity condition on the unknown
functions f, g, h, ...