... Isac, and Th M Rassias, Stability of Functional Equations in Several Variables, vol 34 of Progress in Nonlinear Differential Equations and Their Applications, Birkh¨ user Boston, a Boston, Mass, ... Boston, Mass, USA, 1998 [2] J Acz´ l and J Dhombres, Functional Equations in Several Variables, vol 31 of Encyclopedia of e Mathematics and Its Applications, Cambridge University Press, Cambridge, ... Journal of Inequalities and Applications Banach algebra Ꮽ They have shown that if a mapping f : X → Ꮽ satisfies f (x ◦ y) − f (x) f (y) ≤ (3) with some > 0, then there exist a commutative C ∗ -algebra...