In the paper we consider the stability of the functional equation f ( x + y) + f ( x - y) + ||f ( x + y) - f (x - y)||u = 2f(x) + 2f(y), where f : G —> X, X is a normed space, u ϵ X such that ||u|| ≥ 1 and ||u|| ≠ 3 is ...

We prove the Hyers-Ulam type stability of the Wigner equation for complex Hilbert spaces.

Let {A[s] : s ≥ 0} be a given iteration semigroup of additive set-valued functions. In this paper we study superadditive set-valued solutions Ф of the functional equation Ф(t + s) = A[s] [Ф(t)] + Ф(s) for t, s ≥ 0.