
Summary: Using the fixed point method, we prove the Ulam-Hyers stability of the orthogonally additive and orthogonally quadratic functional equation \[ \begin{multlined} f\left(\frac{x}{2}+y\right) + f\left(\frac{x}{2}-y\right) + f\left(\frac{x}{2}+z\right) + f\left(\frac{x}{2}-z\right) \\ = 3f(x) - 1 f (- x) + f(y) + f (- y) + f(z) + f (- z) \end{multlined} \tag{0.1} \] for all \(x,y,z\) with \(x \perp y\), in orthogonality Banach spaces and in non-Archimedean orthogonality Banach spaces.
Systems of functional equations and inequalities, Operator theory over fields other than \(\mathbb{R}\), \(\mathbb{C}\) or the quaternions; non-Archimedean operator theory, orthogonally additive and orthogonally quadratic functional equation, Ulam-Hyers stability, Fixed-point theorems, fixed point, Functional analysis over fields other than \(\mathbb{R}\) or \(\mathbb{C}\) or the quaternions; non-Archimedean functional analysis, Functional equations for functions with more general domains and/or ranges, Non-Archimedean analysis, orthogonality space, Special maps on metric spaces, non-Archimedean normed space
Systems of functional equations and inequalities, Operator theory over fields other than \(\mathbb{R}\), \(\mathbb{C}\) or the quaternions; non-Archimedean operator theory, orthogonally additive and orthogonally quadratic functional equation, Ulam-Hyers stability, Fixed-point theorems, fixed point, Functional analysis over fields other than \(\mathbb{R}\) or \(\mathbb{C}\) or the quaternions; non-Archimedean functional analysis, Functional equations for functions with more general domains and/or ranges, Non-Archimedean analysis, orthogonality space, Special maps on metric spaces, non-Archimedean normed space
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