On the hyperstability of the Drygas functional equations

Abstract

In this paper we study the hyperstability of the Drygas functional equation of the form
f(x+y) +  f(x-y) = 2f(x) + f(y) + (-y)
in quasi-normed spaces, where f is a map from X into Y and x, y, x+y, x-y ϵ X. The obtained results are the extensions of the results of Aiemsomboon and Sintunavarat (2016) on the Drygas functional equation in normed spaces.