Approximate property of a functional equation with a general involution

Abstract

In this paper, we prove the Hyers-Ulam stability of the functional equation f(x + y, z + w) + f(x + σ(y),z + τ(w)) = 2f(x, z) + 2f(y, w), where σ, τ are involutions.

Keywords

• approximation
• involution
• Banach space