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Stability of a functional equation of Drygas. (English) Zbl 1022.39028
This paper is devoted to establishing the Hyers-Ulam stability of the functional equation $$f(x+ y)+ f(x-y)= 2f(x)+ g(2y)$$.

##### MSC:
 39B82 Stability, separation, extension, and related topics for functional equations
##### Keywords:
quasi-inner-product space; Hyers-Ulam stability
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