Generalized Orthogonal Stability of Some Functional Equations

We deal with a conditional functional inequality x ⊥ y ⇒ ‖ f (x + y)− f (x)− f (y) ‖ ≤ (‖ x‖ + ‖ y‖ ), where ⊥ is a given orthogonality relation, is a given nonnegative number, and p is a given real number. Under suitable assumptions, we prove that any solution f of the above inequality has to be uniformly close to an orthogonally additive mapping g, that is, satisfying the condition x ⊥ y ⇒ g(x+ y)= g(x) + g(y). In the sequel, we deal with some other functional inequalities and we also present some applications and generalizations of the first result.