# Stability of a Mixed Type Additive, Quadratic, Cubic and Quartic Functional Equation

• M. Eshaghi-Gordji
• S. Kaboli-Gharetapeh
• M.S. Moslehian
• S. Zolfaghari
Chapter
Part of the Springer Optimization and Its Applications book series (SOIA, volume 35)

## Abstract

We find the general solution of the functional equation
$$\begin{array}{l} D_f {\rm{(}}x,y{\rm{)}}\,\,{\rm{: = }}f{\rm{(}}x + {\rm{2}}y{\rm{)}} + f{\rm{(}}x - {\rm{2}}y{\rm{)}} - {\rm{4[}}f{\rm{(}}x + y{\rm{)}} - f{\rm{(}}x - y{\rm{)]}} - f{\rm{(4}}y{\rm{)}} + {\rm{4}}f{\rm{(3}}y{\rm{)}} \\ - {\rm{6}}f{\rm{(2}}y{\rm{)}} + {\rm{4}}f{\rm{(}}y{\rm{)}} + {\rm{6}}f{\rm{(}}x{\rm{) }} = {\rm{ 0}}{\rm{.}} \\ \end{array}$$
in the context of linear spaces. We prove that if a mapping f from a linear space X into a Banach space Y satisfies f(0)=0 and
$$\|D_f(x,y)\|\leq\epsilon \quad (x,y\in X),$$
where ε > 0, then there exist a unique additive mapping $$A:X\to Y,$$ a unique quadratic mapping $$Q_1:X\to Y,$$ a unique cubic mapping $$C:X\to Y$$ and a unique quartic mapping $$Q_2:X\to Y$$ such that
$$\|f(x)-A(x)-Q_1(x)-C(x)-Q_2(x)\|\leq\frac{1087 \epsilon}{140}\quad \forall x\in X.$$

## Keywords

Banach Space Functional Equation Positive Real Number Real Vector Space Ulam Stability
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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## Authors and Affiliations

• M. Eshaghi-Gordji
• 1
• S. Kaboli-Gharetapeh
• 2
• M.S. Moslehian
• 3
• S. Zolfaghari
• 1
1. 1.Department of MathematicsSemnan UniversitySemnanIran