Hyers-Ulam stability; hyperstability; quadratic functional equation; fixed point theorem
DE GRUYTER POLAND SP ZOO
DEMONSTRATIO MATHEMATICA, v. 51, no. 1, page. 295-303
We have proved the Hyers-Ulam stability and the hyperstability of the quadratic functional equationf(x + y + z) + f(x + y - z) + f(x - y + z) + f(-x + y + z) = 4[f(x) + f(y) + f(z)]in the class of functions from an abelian group G into a Banach space.