Lie bracket derivation-derivations in Banach algebras

Abstract: In this paper, we introduce and solve the following additive-additive $(s,t)$-functional inequality \begin{eqnarray}\label{0.1} && |g\left(x+y\right) -g(x) -g(y)| +| h(x+y) + h(x-y) -2 h(x) | && \le \left|s\left( 2 g\left(\frac{x+y}{2}\right)-g(x)-g(y)\right)\right|+ \left|t \left( 2h\left(\frac{x+y}{2}\right)+ 2h \left(\frac{x-y}{2}\right)- 2h (x)\right) \right| , \nonumber \end{eqnarray} where $s$ and $t$ are fixed nonzero complex numbers with $|s| <1$ and $ |t| <1$. Using the direct method and the fixed point method, we prove the Hyers-Ulam stability of Lie bracket derivation-derivations in complex Banach algebras, associated to the additive-additive $(s,t)$-functional inequality (\ref{0.1}) and the following functional inequality \begin{eqnarray} \label{0.2}| g, h-g,h y- x g,h | +| h(xy) - h(x) y -x h(y) | \le \varphi(x,y). \end{eqnarray}