On Stability and hyperstability of Drygas functional equation in
quasi-Banach space
Abstract
In this paper, we investigate the Ulam-Hyers-Rassias stability for the
Drygas functional equation $ f( x + y ) + f( x - y ) = 2f (x) + f (y)+
f(-y) $ in the setting quasi-Banach spaces using the approach of
Brzdekās fixed point theorem. Also, we gave a general result on the
hyperstability of Drygas functional equation. The results obtained in
this paper extend various previously known results to the setting of
quasi-Banach space.