The functional equation 𝑓(𝑥+𝑦) - 𝑓(𝑥) - 𝑓(𝑦) = 𝑔(𝑥, 𝑦) has a solution 𝑓 that belongs to C0(ℝ) if and only if the symmetric cocycle 𝑔 belongs to C0(ℝ2). If the symmetric cocyle 𝑔 is recursively approximable, there exists a solution 𝑓 which is recursively approximable also. If 𝑔 belongs to C1(ℝ2) then there exists an integral expression in 𝑔 for a solution 𝑓 that belongs to C1(ℝ), and the same happens for the classes Ck, C∞, analytic and polynomial.