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 equationf(2x+y)+f(2x−y)=f(x+y)+f(x−y)+2f(2x)−2f(x)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.