
Two Pexiderized versions of the quadratic functional equation are considered: \[ \begin{aligned} f(x\ast y)+f(x\ast y^{-1})&=2g(x)+2g(y),\\ f(x\ast y)+g(x\ast y^{-1})&=2h(x)+2k(y), \end{aligned} \] where \(f,g,h,k\) are functions mapping a group \((G,\ast)\) into a~real or complex topological vector space~\(X\). The main results are obtained for functions mapping a~vector space~\(V\) into a~Banach space~\(X\). It is shown that if the remainder \(f(x+y)+f(x-y)-2g(x)-2g(y)\) is bounded in some sense, then there exists exactly one quadratic function \(Q:V\to X\) such that \(f\) is close (in the same sense) to \(f(0)+Q\) and \(g\) is close to \(\frac{f(0)}{2}+Q\). It is also shown that if the remainder \(f(x+y)+g(x-y)-2h(x)-2k(y)\) is bounded (in some sense), then there exist exactly one quadratic function \(Q:V\to X\) and two unique additive functions \(T,T':V\to X\) such that \(f\) is close (in the same sense) to \(f(0)+Q+T\), \(g\) is close to \(g(0)+Q+T'\) and \(\frac{h+k}{2}\) is close to \(\frac{f(0)+g(0)}{4}+Q+\frac{T}{2}\).
Quadratic function, Pexiderized quadratic equation, Banach space, Applied Mathematics, Stability, separation, extension, and related topics for functional equations, Functional equations for functions with more general domains and/or ranges, Functional inequalities, including subadditivity, convexity, etc., Hyers-Ulam-Rassias stability, quadratic functional equation, Analysis
Quadratic function, Pexiderized quadratic equation, Banach space, Applied Mathematics, Stability, separation, extension, and related topics for functional equations, Functional equations for functions with more general domains and/or ranges, Functional inequalities, including subadditivity, convexity, etc., Hyers-Ulam-Rassias stability, quadratic functional equation, Analysis
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