Stability of a functional equation deriving from quadratic and additive functions in quasi-Banach spaces

In this paper we establish the general solution of the functional equation f (2x +y) +f (2x −y) = f (x + y)+ f (x −y) +2f (2x) −2f (x) and investigate the Hyers–Ulam–Rassias stability of this equation in quasi-Banach spaces. © 2007 Elsevier Inc. All rights reserved.