Lipschitz Stability of the Cauchy and Jensen Equations

Let G be an amenable metric semigroup with nonempty center, let E be a reflexive Banach space, and let ?: G → E be a given function. By C?: G × G → E we understand the Cauchy difference of the function /, i.e.: $$ {cal C}f(x,y):=f(x+y)- f(x)- f(y) {rm for} x,yin G. $$ We prove that if the function C(f) is Lipschitz then there exists an additive function A: G → E such that f ? A is Lipschitz with the same constant. Analogous result for Jensen equation is also proved. As a corollary we obtain the stability of the Cauchy and Jensen equations in the Lipschitz norms.