The Stability of Isometries on Restricted Domains

We will prove the generalized Hyers–Ulam stability of isometries, with a focus on the stability for restricted domains. More precisely, we prove the generalized Hyers–Ulam stability of the orthogonality equation and we use this result to prove the stability of the equations ∥f(x)−f(y)∥=∥x−y∥ and ∥f(x)−f(y)∥2=∥x−y∥2 on the restricted domains. As we can easily see, these functional equations are symmetric in the sense that they become the same equations even if the roles of variables x and y are exchanged.