Stability of a functional equation of Deeba on semigroups
Let $S$ be a semigroup and $X$ a Banach space. The functional equation $ϕ(xyz)+ ϕ(x) + ϕ(y) + ϕ(z) = ϕ(xy) + ϕ(yz) + ϕ(xz)$ is said to be stable for the pair $(X, S)$ if and only if $f: S\to X$ satisfying $\| f(xyz)+f(x) + f(y) + f(z) - f(xy)- f(yz)-f(xz)\| \leq δ$ for some positive real number $δ$ and all $x, y, z \in S$, there is a solution $ϕ: S \to X$ such that $f-ϕ$ is bounded. In this paper, among others, we prove the following results: 1) this functional equation, in general, is not stable on an arbitrary semigroup; 2) this equation is stable on periodic semigroups; 3) this equation is stable on abelian semigroups; 4) any semigroup with left (or right) law of reduction can be embedded into a semigroup with left (or right) law of reduction where this equation is stable.