LAUSR

For any fixed $$s \in \left\{ z \in \mathbb {C} : z \ne 0 \text { and } |z| <1 \right\} ,$$ we consider the following functional inequality: 1 $$\begin{aligned}&\nonumber \Vert f(a+a', c+c') + f(a+a', c-c') + f(a-a', c+c') + f(a-a', c-c')\nonumber \\&\quad -4f(a,c)-4f(a,c')\Vert \le \Bigg \Vert s \Bigg (2f\left( a+a', c-c'\right) + 2f\left( a-a', c+c'\right) \nonumber \\&\quad - 4f(a,c )-4f(a,c')+ 4 f(a',c')\Bigg )\Bigg \Vert . \end{aligned}$$ In this paper, we obtain the Hyers–Ulam stability of the proposed functional inequality using the direct and fixed point methods.