Alternative quadratic functional equation on 2-divisible abelian groups

Abstract

Let (G+) be a 2-divisible abelian group and let (Y+) be a real (or complex or rational) linear space. We will show that the alternative quadratic function equation ʄ(x+y) + ʄ(x-y) = 2ʄ(x)±2ʄ(y) is equivalent to the classical quadratic functional equation ʄ(x+y) + ʄ(x-y) = 2ʄ(x)+2ʄ(y) for the class of functions ʄ : G→Y