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On the stability of quadratic functional equations. (English) Zbl 1146.39045
Summary: Let $X,Y$ be vector spaces and $k$ a fixed positive integer. It is shown that a mapping $f(kx+y)+f(kx-y)=2k^{2}f(x)+2f(y)$ for all $x,y\in X$ if and only if the mapping $f:X\rightarrow Y$ satisfies $f(x+y)+f(x-y)=2f(x)+2f(y)$ for all $x,y\in X$. Furthermore, the Hyers-Ulam-Rassias stability of the above functional equation in Banach spaces is proven.

39B82Stability, separation, extension, and related topics
39B52Functional equations for functions with more general domains and/or ranges
Full Text: DOI EuDML
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