A fixed point approach to the hyperstability of Drygas functional equation in metric spaces

Abstract

Piszczek and Szczawi´ nska proved the hyperstability of the Drygas functional equation in Banach spaces. Using the fixed point method, we prove the hyperstability of the Drygas functional equation f(x+y)+ f(x−y) =2f(x)+ f(y)+ f(−y), in the class of functions from a commutative group into a commutative complete metric group.