IMO 1985 LL VIE96
Determine all functions f : R oR satisfying the following two
IMO 1985 LL VIE96
Origin: VIE
Problem
Determine all functions f : R \toR satisfying the following two conditions: (a) f(x + y) + f(x −y) = 2f(x)f(y) for all x, y \inR, (b) limx\to\inftyf(x) = 0.