ABBAS NAJATI - Faculty of Science, Department of Mathematics, University Of Mohaghegh Ardebili, Ardebili, Iran.
FRIDOUN MORADLOU - Faculty of Mathematical science, University of tabriz, Iran
REZA AHMADI

In this paper we estabilish the general solution of the functional equation f (2x+y) + f(x-2y) = 2f(x+y) + 2f(x-y) + f(-x) +f(-y) and investigate the Hyers- Ulam- Rassias stability of this equation in quasi- Banach spaces. The concept of Hyers- Ulam- Rassias stability originated from Th. M. Rassias stability theorem that appeared in his: On the stability of the linear mapping in Banach spaces, Proc. Math. Soc. 72(1978), 297-300.

Hyers- Ulam- Rassias stability, Quadratic function, additive function, quasi-Banach space, p-Banach space