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On the stability of 3-dimensional quadratic functional equation. (English) Zbl 0967.39011
Author’s abstract: We investigate the Hyers-Ulam-Rassias stability of a quadratic functional equation \[ f(x+y+z) +f(x-y)+ f(y-z)+ f(z-x)= 3f(x)+3 f(y)+3f(z) \] and prove the Hyers-Ulam stability of the equation on restricted (unbounded) domains.

39B82 Stability, separation, extension, and related topics for functional equations
39B52 Functional equations for functions with more general domains and/or ranges
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