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Nonlinear stability of a quadratic functional equation with complex involution

Reza Saadati, Ghadir Sadeghi (2011)

Archivum Mathematicum

Let X , Y be complex vector spaces. Recently, Park and Th.M. Rassias showed that if a mapping f : X Y satisfies f ( x + i y ) + f ( x - i y ) = 2 f ( x ) - 2 f ( y ) for all x , y X , then the mapping f : X Y satisfies f ( x + y ) + f ( x - y ) = 2 f ( x ) + 2 f ( y ) for all x , y X . Furthermore, they proved the generalized Hyers-Ulam stability of the functional equation () in complex Banach spaces. In this paper, we will adopt the idea of Park and Th. M. Rassias to prove the stability of a quadratic functional equation with complex involution via fixed point method.

On Functions with the Cauchy Difference Bounded by a Functional

Włodzimierz Fechner (2004)

Bulletin of the Polish Academy of Sciences. Mathematics

K. Baron and Z. Kominek [2] have studied the functional inequality f(x+y) - f(x) - f(y) ≥ ϕ (x,y), x, y ∈ X, under the assumptions that X is a real linear space, ϕ is homogeneous with respect to the second variable and f satisfies certain regularity conditions. In particular, they have shown that ϕ is bilinear and symmetric and f has a representation of the form f(x) = ½ ϕ(x,x) + L(x) for x ∈ X, where L is a linear function. The purpose of the present...

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